I mounted a hot shoe adapter with PC jack on the camera to fire a ring flash. No other flash is mounted on the adapter.
The camera will not let me set manual settings. It says the flash power is not on. Obviously there is not power monitor with the PC jack so how do I fool the camera to let me use it in M mode?
I have exactly the same problem and even with the iso set to 100 it will not work. Is there anything else you did to get it working? I have the hotshoe adapter as you do and am trying to fire a studio strobe with my t5i. I am using manual setting.
Are you using Live View?
Most 3rd party non-ETTL flash units or triggers wil not work in Live View. The camera wil not send a signal to flash because of the way the shutter operates in Live View. "Silent Shooting" is the default in Live View and this prevents the flash from firing. Canon brand or 3rd party ETTL flash units are able to automatically disable Silent Shooting.
Higher end cameras have a menu option to disable Silent Shooting, but the Rebel series don't have this option.