Trying to Install Xmir on Ubuntu 15.04 14.10 14.04 13.04 12.04

1)
Removing the system-compositor-testing PPA

Should you have been using the system-compositor-testing PPA you will have to remove the PPA and it's content from your system prior to installing XMir from archive.

$> sudo apt-get update
$> sudo apt-get install ppa-purge
$> sudo ppa-purge ppa:mir-team/system-compositor-testing
$> sudo rm /etc/apt/preferences.d/50-pin-mir.pref

Then, follow "Installing from Archive".

2)
Installing from Archive

Prior to landing Mir and related packages in the Ubuntu archive, we offered XMir via the system-compositor-testing ppa (ppa:mir-team/system-compositor-testing). As of August 9, 2013 XMir can be installed directly from archive via

$> sudo apt-get update
$> sudo apt-get dist-upgrade
$> sudo apt-get install unity-system-compositor
$> sudo restart lightdm



3)
Verifying that unity-system-compositor is running

You can check in 3 ways whether unity-system-compositor is running:

$> ps aux | grep unity-system-compositor
$> grep -i xmir /var/log/Xorg.0.log
$> ls -l /var/log/lightdm/unity-system-compositor.log

Ref Site: https://wiki.ubuntu.com/Mir/Installing