this is how i got mine to work

1. first store the line command below to /etc/apt/sources.list.d/mene.list and save it
deb http://archive.mene.za.net/raspbian wheezy contrib

2.Import the archive signing key:

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-key 5243CDED

3. Update the package lists:

sudo apt-get update

4.You can then install it as you would with any other package, for example, with apt-get:

sudo apt-get install xbmc

it should install afterwards..

Go to es_systems.cfg in etc/emulationstation and add this command line
<fullname>Apps</fullname>
<name>apps</name>
<path>~/RetroPie/roms/apps</path>
<extension>.sh</extension>
<command>bash %ROM%</command>
<platform>apps</platform>
<theme>appS</theme>

now you would have to create a apps folder
cd ~/RetroPie/roms
mkdir ~/RetroPie/roms/apps

5. Now all you have to do is create a file called xbmc.sh and add the line

#!/bin/bash
sudo xbmc

save it and put it the apps folder you created earlier.
Dont forget to go to terminal and run sudo chmod +x xbmc.sh

thats it..sudo and reboot

let me know how it goes