You're confusing fields and controls. The field is what stores the data. The control displays the data to the user and allows for data entry. If you want a drop down control on a custom form, add it from the control toolbox, then bring up the Properties dialog for the control and, at the top of the Value tab, choose what field you want that control to display.

FYI, there is a newsgroup specifically for Outlook forms issues "down the hall" at microsoft.public.outlook.program_forms or, via web interface, at http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.outlook.program_forms