Basically, install Plex Media Server
(there's a Plex Media Center, which is basically just a Media Player to play content from the Plex Media Server on a PC, so don't install Media Center).
You then sign up for a MyPlex account.
Then you go into the Roku Channel Store (better to go on the website via PC, so you can search easier). Install Plex Channel on Roku. Then you setup the Plex Channel with your MyPlex account.
Now on the computer go to https://my.plexapp.com/queue
and click on "Add Video". Drag and drop the "Plex It!" button to your Bookmarks/Favorites bar. When you see a video you want to watch on Roku, just click on that Bookmark, it will add the stream to your Plex Queue (assuming it's compatible). Then you should see it in your "Queue" on the Plex Channel on the Roku.
Can't advise you on the other channel on this forum, you will have to get support from their forums. Also, not a bad idea if you have trouble with Plex to ask in their forums either.