If you are just starting to setup your box, its recommended that you download a fresh Mythbuntu image
and start from scratch. However, one of the best features of Mythbuntu is that all development happens directly in the Ubuntu archives. This means that you can add all Mythbuntu roles to an existing Ubuntu box.
Click this link to install the Mythbuntu Control Centre, a utility designed to assist in installing and configuring a MythTV on Ubuntu:
Choose the Unity dash and search for Mythbuntu Control Centre to open up the Control Centre. From here you can modify all options that are already setup during a normal Mythbuntu Installation. If you need to setup a backend, this is where you will activate that.