Hi Wasim,
As mentioned by Amit, you are looking at the wrong place for Account Assignment Category..
Are you attaching material to the Network Activities and then want to process the PR generated by the System to PO and GR.
Or you want to create the PR/PO independently using ME51N and ME21N. If you want to directly raise a PR/PO against a WBS or Network Activity, In the ME51N or ME21N you have a field as Account Assignment Category choose Q in there and rest will be taken care of.. 
Once you do the GR for such a PO the material will be in your project stock, which you can see from MBBS or MD51 etc reports..
If its the first case, then check for the following configurations in your system before attaching materials to activities and releasing them.
1. Check T-Code OPTT for relevant settings for document type. By Default it is NB, if your MM counterparts have used different doc. types then you may have to do an enhancement to take the correct doc. type and use the correct number ranges for PR generated from Projects..
2. Check Settings in OPS8 for Procurement Indicators.
3. Check Network Profile Settings in OPUU that same procurement type is being used which you have defined..
Hope it helps..
Regards
Sammar