Can any one tell me what is MM closing and what is FI closing?  How can I know if period is closed?

MM closing means that you can not post materials document. In MM area you have two open periods current and previous. 
If you do not set other in MMRV - here you can check which are the open MM periods for company code. If you check Allow posting to previous period - it will forbid MM posting in previous period. 

FI open period (ob52) allow to post FI documents. As I know you can allow postings for some accounts - for example forbid material accounts for previous period in order to prevent MM posting until you finish all FI documents.

How to add an attachment to a Purchase Order in SAP?

You can attach manually any document to PO without using Document Management System ( DMS ) in SAP 4.6C.

But you cannot attach document while you create PO in T/Code ME21N 
Save you PO ( ME21N ) and again open with change ( ME22N ) then you can attach document through Service for Object button. Service for Object button available down side of Command screen ( in your PO screen ME22N ) 
Click your Service Object button -> Create -> Create attachment 
then select your window directory ( which file you need to attach) select your file Now your system shows message  Service "Create attachment" is started. 
Save again your PO. You can see (open) your attachment in same field ( Service for Object ) select and Create and check attachment list. You can attach as many document in your PO.

Note: This is only information for internal or you can print separately this document for sending to vendor ( fax ) System will not print automatically along PO print out.

What is the difference between Blanket Purchase Order and the Framework Order.

In general the Blanket POs are used for consumable materials such as Xerorx papers with a short text (does not need to have Master Record) with Item Category ' B' i.e.limit, where in the PO validity period as well as the limits are to be mentioned for the simplicity of procurement.The doc type is 'FO' (Framework Order).
The GR or Service Entry for the PO are not necessary in case of Blanket POs as well as one need not to mention the account assignment category during creation of PO it can be 'U'-unknown & can be changed at the time of IR.

I have two issues:

1. Auto P.O.: In MMR & Vendor Master Record (Control data) I have checked 'Auto PO'....I have created the Purchase   Req for the Material.....Now I want to know how the automatic PO will come into force...I mean what steps to do after creating PReq.
2. Source List: I have checked 'Source List' In Purchasing view of MMR...Have two vendors for this material for which source list is also maintained(ME01)....While creating PO..the system takes by default the first vendor....I tried to assign the second vendor (made sure that validity period are in range)..but couldn't do so...Can you suggest how the system will tell me to select one of the vendor I want to assign to the PO.
   

1. For Auto PO , yo should proceed as follows:
   • Auto P.O.: In MMR & Vendor Master Record - checked(MM02/XK02)
   • Maintain Source List & select the indicator for source list record as MRP relevent.(ME01)
   • If more than one source list records are generated, make one of  them as fixed.
   • Run MRP , the PR's generated will be preassigned with the source of supply.(MD01)
   • Enter T Code ME59 for automatically creating PO's from PR's.

2. Check if the assigned Source in PR is not fixed in Source List.
3. Check if the info record for source not assigned,  is having valid conditions.

Why system giving me only warning  message instead of error message when I am trying to make GR with a material  which already tick for deletion flag.  Is there any configuration mistake  ?

Through OMCQ , set  message no M7 430 for error.

When I want to settle Framework order. On Information Text, I had a message "Invoicing plan: No message was found for partner 100454/company code 2000" I had investigate that error on message determination and still the error occurs. Maybe I miss the setting in message determination. Can you tell me how to set Framework order Settlement message determination.

According to SAP Notes, when an invoicing plan is settled, you can create a document/e-mail or etc to notify your supplier or internal personnel. The IMG setting (SAP 4.7) can be set in Material Mgmt --> Logistic Invoice Verification --> Message determination.
Another way is if the creation of these docs are not required, turn off the message by setting the message display from 'Error' to 'Information'. The IMG setting can be set in Material Mgmt --> Logistic Invoice Verification --> Define Attributes of System Messages. This is what I did on my side.

Have any one you done this in IM:  Inactivating the standard Movement Types.  Say for eg. I don't want to end user to use this MType 501- Receipt without ref to PO. I don't want to delete this movement type, being an SAP standard one.  One option what I thought of is to control in the authorisation profile, profile to exclued selected movement types.
Is there any other way we can do this?

You can delete the accounts assignments which are used for 501 movement type, in T030 table. Transaction code is OBYC.

or

1. Use transaction OMJJ
2. Enter movement type 501
3. Remove MB01 from allowed transaction