So, to hopefully end ‘silent’ complains that Mono-D can’t be installed anymore — I just added the instruction step which adds the external repository to the system’s software repository list, again.
You have to execute
sudo add-apt-repository ppa:keks9n/monodevelop-latest
sudo apt-get update
sudo apt-get install monodevelop-latest
in order to download MonoDevelop 3.x.x. Aptitude should also be usable to download the latest alpha.