Most of the search results on google are out of date and/or contain links which no longer work.
I also tried compiling everything from source but ran into trouble here too.
Eventually after much headscratching I found a solution :)
Take a look at this page: http://fedoraproject.org/wiki/EPEL
Ignore that this is a fedora project as it works just fine on centos.
Part way down the page under the "How can I use these extra packages?" heading are a couple of links. Choose the one that is appropriate to your version of centos. e.g. I am using centos 6 so I used the "newest version of 'epel-release' for EL6" link.
Download the rpm file and install it.
You should probably verify the rpm before installing, but I couldn't work out how and I was loosing patience by this point!
rpm -i epel-release-*.noarch.rpm
Now you should be able to search for and install the mono packages (and others) using yum.
yum install mono-core mono-data mono-devel mono-extras mono-web mono-winforms monodoc mono-web-devel
N.B. there is no monodevelop included in this lot.
And I can't find anywhere to get it either :(