It depends somewhat on where you are typing the formula.  
For instance, you are typing in the Code window it won't show up.  The same with a query. If you Zoom in (Shift-F2) you will not see it.
On the other hand, if you are typing in the criteria line of a query the dropdown for Forms! should work.  That by the way is called the IntelliSense in MS Access.  

Mike over at No Longer Set published a great article not to long ago.  Check it out.
Quick Tip: Force IntelliSense to Appear When Writing VBA