With our latest release of Pluto (.19) we upgraded to the latest version of MythTV (.18), and there is a bug in our code that it's not properly able to launch Myth setup from within the on-screen Orbiter. Sorry. We did fix this and it will be available in our release (.20), which will hopefully be up within a few days. If you're Linux/tech saavy, you can try to launch it manually. You normally only need to run MythSetup once--from then on the 'watch tv' button should work. To launch it manually, ssh in to the machine and run:
# that will start Myth setup, now you need to activate the window so it's topmost
./ratpoison -d :0 -c windows
# you will see a list of windows, make note of the number for mythsetup, and put that number instead of ??
./ratpoison -d :0 -c "desktop off"
./ratpoison -d :0 -c "select ??"
./ratpoison -d :0 -c "desktop on"
# Now you should have mythsetup. Configure everything and exit like normal. Leave it for about 2 minutes so it has enough time to start pulling guide data, and then you should reboot the core/hybrid (and any md's). That's not related to this issue--the myth master back-end always needs to be reset after changing the config of the front-end's, and we've found that manually restarting it, it crashes a lot. so we always recommend a full reboot after running mythsetup.