from "What devices are supported?" on 7/07/2012, playon.tv/supporteddevices,
In order to get the PlayOn channel on your Roku, go to https://owner.roku.com/Add/MYPLAYON and add the private channel code MYPLAYON. Follow the simple instructions that will be provided once the code is entered and that's all there is to it. Now sit back and enjoy PlayOn on your Roku!