I would assume since they mentioned the VIDEO app on the iPad, it's happening with that app.
OP, So you're trying to send video from the Home Sharing menu on your iPad to the TV? If you have an Apple TV to accept AirPlay, you can leave the iPad out of the equation and just send the videos to your ATV from the iMac. Sorry, I'm trying to understand this so I can try to be of assistance.
"I am trying to AirPlay videos from my iMac via the shared menu in the video app on the ipad ( hope that made sense)."