I installed MonoDevelop from Add/Remove... which did not install all addins I wanted.
Basically I was searching how to install NUnit addin for MonoDevelop. There's a FAQ entry on how to enable it, but it is not very useful, though I did the last bit, creating symbolic link.

Anyway, here's how to install it on Ubuntu:

sudo apt-get install monodevelop-nunit

If you just type

sudo apt-get install monodevelop-

and press Tab key twice it will list you all available addins. One very useful is monodevelop-versioncontrol,

Useful links:
Using NUnit with MonoDevelop
Using Subversion with MonoDevelop