From the looks of it, you're running into the following bug:
CQ00312474: Fields for external list values only show on XP machines (not on Win7/Vista)
When you are utilizing a dbf table to populate the drop-down lists of a combobox control it requires that you select both the Values and Text fields (Code and Description) from the dbf table.
Unfortunately, these lists appear blank on Windows Vista and Windows 7 computers. Until this issue is resolved, you MUST select these values via ArcPad Studio on a computer with a Windows XP operating system.