Getting your source on is a whole lot easier when using a package manager, for linux you might use Yum or APT, but on OSX Homebrew is my goto. It’s super easy to install, but to get full benefits and access to more software you will need to have an install disc or have the developer tools already installed. If you don’t you can download it from apple. Google it or comment if you need to know what I’m talking about.
Here’s how you install Homebrew:
ruby <(curl -fsSkL raw.github.com/mxcl/homebrew/go)
of course using ruby is awesome, because it's installed by default on everyone's machine and has read write. Also notice that it's not using wget, which you won't have on a clean install w/out the dev tools. Underneath brew is pretty much an alias to GIT with some cool ruby helpers.
Once you have brew installed you can do things like:
brew install mongodb