I have added mono 2.10 using some great information I can not provide a link to (integratedwebsystems.com/2011/05/install-mono-2-10-2-and-monodevelop-2-6-beta-3-on-ubuntu-or-fedora-with-a-bash-script/comment-page-1/#comment-6975)

I think it went ok, so now I have mono 2.10 in /opt/mono-2.10/

But my desktop machine also comes with a built-in version of Mono, and when I try to run a website with the 2.10 XSP by executing "/opt/mono-2.10/bin/xsp2 --root [pathtositeroot]" I get an error saying:
Description: Error compiling a resource required to service this request. Review your source file and modify it to fix this error.Compiler Error Message: : ** (/usr/lib/mono/2.0/gmcs.exe:3595):

and the thing here is that it uses the compiler from the built-in version in /user/lib, instead of using the one from /opt/mono-2.10

How can I force mono to use the mono-2.10 things instead of the old, built-in ones?