Is Mono installed in 13.04?
I just checked my installation of 13.04, and Mono is not installed.
To install Mono, this will install Mono Run-time, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo apt-get install mono-runtime
If mono Develop is what you're after, then
sudo apt-get install monodevelop
Mono focuses on compilers and run-time libraries and does not directly provide an IDE. MonoDevelop is a cross platform IDE mostly aimed at Mono/.NET developers. For more info see MonoDevelop, and Mono
It's easiest to install using the Ubuntu Software Center. Just search for "mono" in Ubuntu Software Center and you can install it from there. It's called "MonoDevelop". You can also use this link: http://apt.ubuntu.com/p/monodevelop