Install OSX and the Mythtv frontend on a clean system. 
For Christmas this year I got an awesome gift of a Mac Mini core solo to use as a Mythtv frontend on my bedroom entertainment center. My brother had recently bought two of them and said they worked great. He configured the Mac against his own Mythtv system in Ohio and then shipped it too me in Florida. The unit booted fine when I got it but would not connect to my Mythtv backend. So I started from scratch and here’s how I did it.
1. Load OSX Tiger on your Mac Mini. This is easy, just load the disc and follow the prompts. When prompted choose “erase and install” to prevent data from a previous install from interfering with your new OS.
2. Go the the Finder, download and install all necessary updates then reboot.
3. Go to http://www.mythtv.org/wiki/index.php/Myth_on_Mac_OS_X. Download version 0.20.2 from the Vanvalkinburgh (Universal Frontend) site.
4. Start up the Myth Frontend application and enter the following:
a. IP address of the Mythtv backend
b. SQL user-name, password (Issued at install of backend), and database name.
That’s it. EASY.
Troubleshooting.
If you have problems connecting your new Mythtv frontend to your Mythtv backend try this:
Sometimes the SQL database access permissions on the backend server get messed up. I have read all sorts of posts on how to fix this and none of them except one worked. Here it is.
Run “sudo dpkg-reconfigure mythdatabase” on your backend machine. (Debian based systems only. I.E. Ubuntu)
This will reset all connections to the database to default just as if you just installed the SQL database. This does not affect any of your previously recorded shows or database entries so nothing will be lost.
Good Luck!