Can't install Xfinity Stream Beta on Roku

I have a Roku 3500 which I've seen on some lists as an approved devices, but not on others. I'm able to see the Xfinity app, but the picture/icon is blank. When I try to install, I get an error message that it can't be installed. So, is the Roku 3500 not a supported device? I've also got 2 smart tvs that can't download it. From seeing all the questions and issues on here, I'm hesitant to buy a new Roku because there's no guarntee that the app will work well even if I can get it installed. I'm not sure what to try next!