Help!! Novice problem!

I am trying to create and distribute a custom form in Outlook. I manage to design the form ok; I can even manage to publish it. The problem starts when I want to make it the default form for the folder I created. When I select my form in the “When posting to this folder” it won’t let me and a box pops up from Microsoft Office saying ?You cannot create an item of this type in this folder” Why?! I hope this isn?t too much of a basic question for here, but I am pulling my hair out on this one. My end result is to have the whole office using this form; I am I fighting a lost battle? Many thanks in advance.