This is quite tricky and my university lecturer took a whole month to explain this one so please bare with me. What you have to do is enter a new macro and go to View -> Conditions. You will know see that a new column has been added. You will now have to enter code similar to the example;
This states that if the value 6 is entered in the field [Size] on the form [EPOS] then do the Action that is documented in the action column. In this case just do the usual Open form e.t.c. Hope thats pretty clear enough! I will post an example of the macro so you can see for yourself!