Docs now up to date as of version 3.1 (July 14th, 2013).
Xtra is very easy to install. One simple cut and paste command via SSH gets and installs the files for you. Xtra is accessible via any WHM theme from the Plugins Menu at the bottom Left of the WHM Menu area.
Once installed you should then log into WHM and click on the Xtra Ultimate link. Enter the license key in the box and click the change button. Once the setup completes click the link to go to Xtra. That's it, you're all set to start using WHMXtra Ultimate Pro Suite.
Note: Xtra is encoded with Ioncube so before using it you should ensure that the php loader under WHM's Tweak Settings section is set to Ioncube.
If you need to uninstall Xtra you can do so via SSH by simply logging in and typing the following commands:
wget http://whmxtra.com/pro/remitall.sh sh remitall.sh
Caution: This will remove Xtra completely, including all custom links and settings.