I installed WLM 8 and its working fine.
But is there any way that I could get the details of contacts which I had saved on Messenger Plus! before upgrading??

Put simply, no. Plus! features don't work with WLM.

i dont want it to WORK with WLM. I just want to export the details and then I'll add them in Manually.

Plus! does not work with WLM yet, so there is nothing to import or to add them to in Plus!...

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Unless you mean adding the Plus! contact information manually into the new contact information supported by WLM. In that case:

The field where you can enter individual contact information in Plus! is stored in the registry per account per contact under:

HKEY_CURRENT_USER\Software\Patchou\MsgPlus2\your .NET Passport\Contacts\encrypted contact .NET_Passport\Notes

There is no method (in Plus!) to mass-export only this into a readable textfile with associated listed contacts, and there is no information on how to decrypt the contact's email address to directly find the corresponding key. So, you manually need to copy everything from the registry and paste it into the appropiate field in WLM contact per contact.

yeah but isnt there some kind of database or data file that i can open and manually remove the details from??

No,
the field where you can enter individual contact information in Plus! is stored in the registry per account per contact.