First you create your button and your subform,
In the subform, in the object->subform tab, in the presence option, select hidden.
Select your button, and in the script editor, choose formcalc as the language. Then, you select the event Click.
In the editor, you click as if you want to write in it, then you hold CTRL and click on your subform. This will write the path to this subform in the editor. Then after the path you write :
path.presence = 'visible' (don't write majuscule on visible or it won't work)
This should make your subform appear.
Max
