How do you force the 7D to use flash?
Pop-up the built-in to activate it. Or attach a speedlite to the hot shoe and power it on. They should fire regardless of lighting conditions, automatically sensing whether to be main light or fill depending on light level and exposure mode selected.
Double check the 7D's Flash Control Menu to make sure you haven't inadvertently set Flash Firing to Disable.
I've flipped through the manual, checked the menu, and I just can't figure it out. The 7D seems to prefer long exposures to firing the flash. I tried on full auto and aperture priority this evening and on both settings I was getting around 1/10 sec. shutter speeds, but no flash. The one time to flash did fire, the picture was overexposed.
Re - overexposed, my guess this was outdoors in sunshine with the built-in and Av mode. In that situation, the shutter speed is limited to the x-sync speed of 1/250. That combined with chosen aperture gave overexposure. Tv, P, CA, and green box auto modes will have the same shutter speed limitation, but should have been able to stop the aperture down enough to compensate.
Faster shutter speeds with flash require an external speedlite capable of FP High Speed Sync (all of Canon's -EX series speedlites and select third party flashes).