1.wget
2.tar -zxvf ...tar.gz
3.sudo rm -rvf /data/db
4.sudo mkdir -p /data/db
5.sudo ./mongod --dbpath /var/lib/mongodb/ --port 12345 or ./mongod
6.
reference:
http://www.mongodb.org/display/DOCS/Starting+and+Stopping+Mongo
http://www.mongodb.org/display/DOCS/Quickstart#Quickstart-32bit