The user account that is starting MongoDB needs to have write privileges to the /data/db/ directory … you can change this like so …
This command will change the owner of the /data/db/ to the user account being used.
$ sudo chown `id -u` /data/db
$ mongod
If you still run into problems with MongoDB starting, you need to delete /data/db/mongod.lock and then run repair on your database …
mongod --repair
More about the repair command here.