Installation in Debian
Add repository line to your sources.list
and install Xamarin GPG key. The most simple way to do that is using software-properties-gtk or mintsources (in case you are using Mint or LMDE). It also can be done from command line:
sudo -i
echo "deb http://download.mono-project.com/repo/debian wheezy main" > /etc/apt/sources.list.d/mono-xamarin.list
wget http://download.mono-project.com/repo/xamarin.gpg
apt-key add - < xamarin.gpg
Now we have to update apt cache and upgrade packages:
sudo apt-get update
sudo apt-get upgrade
Now get some tea (coffee) and sandwiches and wait… In case of problems check official installation guide.