Now you can trigger your commands from your Roku with the TRIGGERcmd Roku channel. Here's a youtube video showing it in action.
Use this link, or search for triggercmd on your Roku.
The Roku app / channel does not play videos, unless you make a command on your computer that plays a video on your Roku like this:curl -v -XPOST "http://(roku IP address):8060/launch/837?contentId=A7vYpIjfyYE"
That ^ plays this Youtube video on a Roku: