You could do this a couple of ways. The best way I have found is to create a table with the form name in one field and the menu item number in another field. Then your code sets a variable to your form name based on what the value of the list box is.
Dim formname as string
formname = dlookup("[formname]","tblFormNames","[formnumber] = " & forms!frmMenu!listbox)
docmd.openform formname
This code should go in the On Change property of the list box on the form.
Make sure your menu item number box in the table is set as a number or you have to play around with quotes which is not fun in the Dlookup statment.