Mono on Ubuntu
Apparently current Ubuntu (11.04?) included mono runtime support so you probably just need monodevelop.
Packages for Ubuntu 10.4
See http://monodevelop.com/download and click on the Ubuntu icon.
Compiling from source
There are instructions for Ubuntu 9.04 at http://monodevelop.com/Developers/Articles/Building_MonoDevelop_on_Ubuntu
which might form the basis for building on a later version.
Alternatively, find the latest version and replace 0.16 in the commands below:
sudo apt-get install mono-devel build-essential mono-gmcs libmono-dev libpango1.0-dev libgtk2.0-dev libgtksourceview2.0-cil libgecko2.0-cil monodoc libmono-system-runtime2.0-cil libmono-cairo2.0-cil gettext
tar xvf monodevelop-0.16.tar
./configure –prefix=`pkg-config –variable=prefix mono`
sudo make install
-- from HowToGeek
See also Ruski's blog
It all looks a bit of a mess to me - good luck.