Ubuntu 10.04 comes with mono. You can check the mono version using

mono -V

To install MonoDevelop follow the following steps-

1. Click on "System", "Administration", "Software Sources".
2. Click on the "Other Software" tab.
3. Click on "Add...", and enter the line:
   

   deb http://badgerports.org lucid main

   
4. Click on "Add Source"
5. Click on "Authentication", then on "Import Key File"
6. Download this GPG key file, ID 0E1FAD0C, and select it in the "Import Key File" window
7. Click on "Close", then "Reload" when the pop-up appears. You're all set!

From now on, whenever you try to install or update Mono-related packages on your system, you will be given the badgerports version for preference. You don't need to do anything more than update your system as usual to get started!

Then, go to System -> Administration ->Synaptic Package Manager. Find monodevelop from there. Mark for install and press apply.