Unable to prepare input file for HSPF software for watershed modeling!
I have found the solution and it seems that in BASINS 4.1, user has to import wdm file before launching HSPF by going to 'File' and then to either 'import data' or 'manage data'.  Once this step is done, BASINS 'understands' that there is a wdm file and then the user can add the wdm file from 'met stations' tab. 
The tutorials on epa site correspond to BASINS 4.0 and in BASINS 4.0, one doesn't have to go through the so call 'import data' step before launching HSPF.