Rick,

Presuming that the Project Category is bound to a control on your form, one
way is to disable the button by default and enable it when the category has
the special value:

If Me![txtProjectCategory] = YourValue Then
   Me![YourCommandButton].Enabled = True
Else
   Me![YourCommandButton].Enabled = False
End If

Place the code in the Project Category AfterUpdate event and the form's
OnCurrent event.

Alternatively, you could make the decision in the button's OnClick procedure:

If Me![txtProjectCategory] = YourValue Then
   ...open form here
Else
   MsgBox "This form only applies when Project Category equals 5."
End If

Hope that helps.
Sprinks