Here is a quick guide for installing MongoDb for developers.
Download file
curl http://downloads.mongodb.org/osx/mongodb-osx-x86_64-2.4.7.tgz > mongodb.tgz
Extract file
tar -zxvf mongodb.tgz
Move files to usr/local folder
mv mongodb-osx-x86_64-2.4.7/ /usr/local
Open your .bashrc file
cd
open