The backend should already be there if you installed from the Mythbuntu cd, what's more it should already be running. The frontend and backend packages should be available in your package manager.
There's not much point in having the backend on another machine unless you have other computers with the frontend installed and you want a central point they can all access to watch tv, play video files, view pictures, play music etc. And you will need to install and setup a MySQL database!
Shameless Plug Time
: If you're in the mood to try another (in my opinion better) Myth LiveCD distro, I can recommend MythtvOS. Everything works out of the box, both front & backends run in live mode and they also get installed at the same time. If you live in the UK it comes pre-configured for the UK tv stations. You can setup the configuration you want in live mode then install it retaining the same configuration. The support forum is second to none and it leaves you with a warm, fuzzy feeling inside. Have I not convinced you yet?
You can download it from here: http://linux.softpedia.com/get/Syste...OS-34233.shtml
Check out their forum here: http://www.mypclinuxos.com/forum/index.php?board=94.0