Paul and Kunal,
First, make sure that you specify the correct forcefield file (cff91.frc in this case). Second, make sure that your car/mdf files have the correct atom types that correspond to cff91.frc.
If msi2lmp.exe produces a data file with incorrect parameters, then it is likely that cff91.frc does not have parameters for all of your interactions. I have seen this with cvff.frc. In that case you can either adjust the parameters in your data file, or add parameters to cff91.frc.
Hope this helps.