When I enter it says:

The expression you entered contains invalid syntax.
You omitted an operand or operator, you entered an invalid character or comma, or you entered text without surrounding it in quotation marks.

But when I open Dmax in the control source it gives 3 fields instead of 2.
And I am sure my table and field is correct and I placed the quotation marks.

Also when I change to the main page of access and then back to the form, the control source i typed, has dissapeared.

greetz