You will have to run mythsetup (on your orbiter go to Computing>MythSetup). From there go to your Input Sources, select the appropriate one and in its config point it at a suitable channel changing script.
Now, LinuxMCE provides one by default in /usr/pluto/bin/TuneToChannel.sh, however I have had trouble with that these past couple of days (look at my USB-UIRT learning thread). With a couple of modifications to the script itself to remove DOS carriage returns I could get it to blast channel change requests, but every button press was repeated 4 times, making it ineffective.
Sadly, I can't help much further with getting the channel changing script to work with the USB-UIRT. The script just calls SendMessage which is a binary and I'm not familiar with how it works. As yet, no gurus have responded to my thread with further info. I was hoping I could pass a parameter to it from the channel change script to have it not repeat button "presses", but no luck so far.
In the interim, I returned to working on getting lirc to control the irblaster built into my MCE receiver; and I have been successful. I will post with links to wiki pages once I get them updated.