I’ve been able to watch content from my Tablo on my iMac but now I’m trying to figure out how to have that content shown on my Apple TV via AirPlay from my iMac. So far, no luck. I’ve tried on both Safari and Chrome but do not see an affordance for selecting AirPlay. Any suggestions?
(I’ve been able to play content from my iPad on my Apple TV just fine.)
I’m pretty sure you can only AirPlay the Tablo from iOS devices, so yes your computer with the browser will not work.
Your Mac has to support AirPlay. You can find it in your display settings. I don’t know about iMacs but with MacBook it wasn’t support until after late 2011 models.
That said…iOS AirPlay is much better. 
If your Mac is new enough, it should be possible. The AirPlay icon should be in your menu bar at the top right. It will AirPlay your entire screen, so you would need to put the content into full-screen mode.
If your Mac is not new enough, or still doesn’t work right, look into an app called AirParrot. It does the same thing when AirPlay is not built in. I think they have a free trial, and it’s cheap to buy.
Thanks everyone for the feedback. It turns out that AirPlaying from this 2015 iMac is only possible from iTunes with the exception that the entire monitor can be mirrored via AirPlay. That, unfortunately, makes it impossible to use the iMac for anything else at the same time.
Thanks for the update!
You should be able to on a 2015 iMac.
On your iMac display in the upper right hand corner you’ll see the “AirPlay logo”.
Click that - turn on AirPlay
Click it again and you’ll get the option to Mirror or Extend your display.
Extend.
You should see your extended display on your AppleTV screen.