I looked at running MongoDB on the Pi several months back, and while you can download the latest source and compile it yourself which takes a while, there didn’t seem to be any prebuilt binaries (at least not when I last looked).
It seems the 32bit version for ARM is now in the Rasbian repos, so you can just do (from here):
sudo apt-get install mongodb-server
and get it installed and setup ready to go.
If you’re looking for a later 64bit version then some people have rebuilt these for you. Take a look at here, or for more info, also try Andy Felong’s posts, e.g. here and here.