If you put the equivalent of following code in the Add button it will work
generically. I use it in a Class that has grid called grdItems.
It works both with 3.0 and 5.0:

FOR ln = 1 TO THIS.Parent.grdItems.ColumnCount
IF !THIS.Parent.grdItems.Columns(ln).ReadOnly
THIS.Parent.grdItems.Columns(ln).SetFocus()
EXIT
ENDIF
ENDFOR