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 .bashrc

Add the following lines to your .bashrc file

export MONGO_PATH=/usr/local/mongodb-osx-x86_64-2.4.7
export PATH=$PATH:$MONGO_PATH/bin

Create data directory

cd
sudo mkdir -p /data/db
sudo chown `id -u` /data/db
ls -ld /data/db