Make sure that you have at least one VPI location specified in the vertical profile. When you're defining the first VPI, the grade option will be grayed out as all you can specify for the first point is the station and elevation.
Once you establish the first one, then the grade option should be available from that point forward.
On the second station (and subsequent stations), on the dialog, pick the little button to the right of GRADE - then you should be able to enter in a grade. It defaults to Elevation for each station and you have to pick on the GRADE button each time you wish to use it.
Cheers,
Glen W. Cameron, C.E.T. City of North Bay, Ontario