What it wants you to do is:
Make a new shortcut to the OFP .exe and in the target field add.... -mod=@modulsky
So, make the shortcut, then right click it and choose Properties.
When the properties dialog is displayed, look for the Target field, it will look something like:
C:\Games\OperationFlashpoint\FLASHPOINTRESISTANCE.EXE
You add -mod=@modulsky onto the end of that line.......not forgetting to have a space after .EXE.......like so:
C:\Games\OperationFlashpoint\FLASHPOINTRESISTANCE.EXE -mod=@modulsky
Planck