The command listed to add the Mono Project GPG signing key failed with "gpg: keyserver timed out". Heading over to the Mono docs on mono-project.com (http://www.mono-project.com/docs/getting-started/install/linux/), we find this command represented a little differently:
Current doc @36:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
Proposed change:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
My Ubuntu VM liked the command above and was able to add the GPG key.
~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.2 LTS
Release: 14.04
Codename: trusty
3ceb59d24e