If it is a Trade or Business, you can go ahead and enter the income on Line 3b of that worksheet.  The program is just not set up for the ability for determining if a rental is a "Trade or Business" or not.

As a side note, in MOST cases you can e-file with an override if you disable error-checking when you e-file (you need to file from Homebase to have that option; it does not give you that option if you e-file from the 'tab' within the client file).  But entering it on Line 3b of that worksheet should do it without any overrides.