AirPort Express not able to complete 'Join Network' - (error message attached)

Added a printer to my basement hooked to an APE (N) and the only way I can get it to print is to 'create new network', which is fine except it forces me to switch from internet (router) to the APE.

The correct way would be to 'Join Existing Network' however just when it's finishing the configuration, I get this dumb error message telling me no dice. Any ideas?

for the record, I'm using a MacPro w/Airport card, modem and router (Rosewill) are in the attic, and the APE is in the basement with the ginormous printer...

thanks!!