Chances are that your video output is not setup correctly for your monitor/TV. Try this:
If you launched LMCE from ubuntu then you can get switch back to Ubuntu world by pressing ctrl-alt-F7. Open a browser and type the IP of your machine. This should bring up the LMCE/pluto admin page. If the left hand column select "Media Director". It will open a new page, and in the lower right hand corner you will be able to change the display resolution. Try different settings, hit update, and then switch back to LMCE to see if it helped. You can switch back to LMCE by pressing ctrl-alt-F11.
If you choose the install option to boot LMCE directly then you will need to access the LCME/pluto admin page from a different PC on your network.
LMCE updates the video configuration by writing to a file called /etc/X11/xorg.conf.pluto. This file is used inside of LMCE. Once you have a working file make sure that you back it up. LMCE regenerates the file each time it starts up. Some people (including me) had difficulting with LMCE not detecting our TV information correctly. If you have the problem where it works initially but then won't work after reboot you can fix it like this. Add the line 'exit 1;' to the top of the /usr/pluto/bin/XConfigure.sh script. That will abort the script early and prevent LMCE from regenerating a bad config file. Most people shouldn't need to do this, but I did.