There's nothing you have to (or can) install -- the Mono environment should be installed already (and it requires access you don't have to install if it's not already installed). If Mono isn't up and running, you'll need to open a support ticket to get it fixed.
That said, be aware that Mono isn't ASP.NET on a Windows server with IIS. It doesn't do all of ASP.NET, and its not always perfectly compatible in the parts it does do. It's just a way of making ASP.NET code run on *nix servers so that programmers can leverage that skill set without needing Windows servers (which may either be too expensive or just not what your IT department supports). It might help to be aware of Mono's limitations before you go much further, so go to the
Mono Project site and check the overview and compatibility documentation. You may have to rewrite parts of your code, or may be trying to use elements of ASP.NET that Mono doesn't support.