Ah, now I understand the references to MCE and port 5150.
You are not alone with what I believe you are hoping for. Which is a RR Remote Control Interface with ideally a Media Guide for MC's Theatre View. This has been requested of the RR / Simple Control developer by a few of us.
However you can still do what you want if you build a custom remote and then assign the following urls to the buttons. (open silently)
Or test it in a browser and if that works, it will work on RR.
UP http://<your MC PC IP>:<your MC port>/MCWS/v1/Control/Key?Key=Up&Focus=1
Down http://<your MC PC IP>:<your MC port>/MCWS/v1/Control/Key?Key=Down&Focus=1
Left http://<your MC PC IP>:<your MC port>/MCWS/v1/Control/Key?Key=Left&Focus=1
Right http://<your MC PC IP>:<your MC port>/MCWS/v1/Control/Key?Key=Right&Focus=1
Enter http://<your MC PC IP>:<your MC port>/MCWS/v1/Control/Key?Key=Enter&Focus=1
More info can be found if you point a browser to http://<your MC PC IP>:<your MC port>/MCWS/v1/ and scroll down to the Control section.
I think you can send pretty much any keyboard command, including some extended ones as well using this method.
Have not actually tried the commands above yet, not at my home at the moment.
I think the default MC port number is 52199, but don't quote me on this.
[EDIT] tried this using Safari to control my MC running in theater view and could navigate using the browser, so this would work just fine with a custom remote design with Silent URL's assigned to the Navigation Keypad layout.
You can then add Play/Pause, skip, rewind etc.