To change your UI you have to access the Web Admin (by typing your Core's IP in a browser) and go to the media directors section. You will find there the UI dropdown, and be able to change it to UI1.

In the Web Admin, choose Media directors in the left side tree, under Devices. Then, you will see Video Settings and resolution (in the rights side), in the bottom, you will also have UI, if you want to switch to another UI too.

If you get this error type in the console:

chroot /tmp/tmp.....
apt-get update
apt-get install pluto-orbiter

after it's finished return to the installation window (the small black window) and press CTRL+D to resume the installation from that point

Did you choose the correct serial port when you added CM11 in the interfaces section?

You can switch to UI1 using the Web Admin configuration page, not only from AV Wizard. You can access this by typing your Core's IP in a browser. In the Media Directors section, you have a drop-down box (the bottom right corner). Change to UI1 and press Update.

What kind of interface did you choose? Try UI1.

You should try running  /usr/pluto/bin/StorageDevices_StatusRadar.sh, or /usr/pluto/bin/StorageDevices_Radar.sh
Note that LinuxMCE scans only your internal network.
this should detect all your storage devices (partitions) that are not already added. You should see the message on your interface. If not, try reloading the router afterwards.
Also, check this out:

http://wiki.linuxmce.com/index.php/Howto:_Using_Network_Shares
http://forum.linuxmce.com/index.php?topic=1347.0

Check this out

http://wiki.linuxmce.com/index.php/Howto:_Using_Network_Shares

you can install these packages when running the wizard, when you get to the 'Add Software' step, or by hand: 'apt-get install libdvdcss' in the console.

As far as I know, PVR x50 works fine.

You can use Squeezebox for that. You should connect it to your internal network and your core will detect it (assuming it has DHCP enabled). Only make sure to put it in a separate room in LinuxMCE, as you do for every other media director.