Hi Sania
I had to learn this one the hard way, so in brief detail you'll need to do the following, all within the LSMW Transaction.
Under Settings - Choose: Idoc Inbound Processing
* You'll need to define: File Port: Maintain a Port named LSMW
tRFC Port: Use the Generate Port Name option
Partner Type (US) - Maintain Partner Type
* This is where you'll specify the Message Type I detailed above
Partner Number - Maintain Partner Numbers
Once these are done select: Activate Idoc Inbound Processing
Once you'll done all of these settings you can build the LSMW processing, go through each step by step...all the way down to Start Idoc Processing...by the end of this you should have BP's being created via Idocs.
This is a very brief summary of the steps you need to follow.
Regards
Arden