After you’ve installed XenTools on your VM and you try to do a apt-get update you are greeted with a GPG error message saying that the repository isn’t trusted. This is because the GPG key Citrix use to sign their packages has not been added to the apt keyring.
To fix it, run;
wget -q http://updates.vmd.citrix.com/XenServer/5.5.0/GPG-KEY -O- | apt-key add —