Did you create your variable in the Variable Designer within Calc Manager? If so, if you look at your variable in calc manager, just to the left, is there one cube, or three little cubes? This indicates whether your variable type is "Member" or "Members" (plural).
My understanding is that you can't pass a run-time-prompt to a business rule if you've selected "Members". Instead I think you have to select "Member".
(Hopefully this isn't what Cameron was referring to . . . I didn't check his link.)
Hope this helps,