I had some issues getting Monodevelop to build on OS X. These occured when I tried to issue configure, and looked something like this:
checking for UNMANAGED_DEPENDENCIES_MONO... no configure: error: Please install mono version 2.4.2 or later to install MonoDevelop. Please see http://www.mono-project.org/ to download latest mono sources or packages
This error message was extant even though I could see from executing mono -V that I had a more recent version of Mono installed and available. The reason behind this seems to be that mono.pc is available by default in:
/Library/Frameworks/Mono.framework/Versions/2.6.1/lib/pkgconfig/mono.pc
However, pkg-config checks (in the absence of an environment variable stating otherwise) the following default path:
/opt/local/lib/pkgconfig
The fix to this is pretty simple, you can either symlink mono.pc into the default path, or export a new environment variable called PKG_CONFIG_PATH with the correct path:
export PKG_CONFIG_PATH=/Library/Frameworks/Mono.framework/Versions/Current/lib/pkgconfig/
This may be sufficient for you to proceed with your build; I had an additional issue, in that glib2 was not installed on my machine. You can identify this by executing pkg-config directly, and checking it's result:
pkg-config --exists --print-errors mono Package glib-2.0 was not found in the pkg-config search path. Perhaps you should add the directory containing 'glib-2.0.pc' to the PKG_CONFIG_PATH environment variable Package 'glib-2.0', required by 'Mono', not found
I used the excellent MacPorts to install this.
If you're hacking on add-ins, like myself, you may need to get Mono.Addins from SVN, as the bundled version (0.4) is not recent enough to build AddinAuthoring (http://go-mono.com/forums/#nabble-td1578892).