Both answers above are correct.
You can either specify the path in one of the following files: .profile, .bashrc, or .bash_profile
export PATH="$PATH:/usr/local/mongodb/bin"
then call the daemon or the shell directly
mongod
mongo
Or for the commands not in the $PATH, use ./mongo or ./mongod from the directory containing these files. This solution can be verbose has you will have to eventually append the whole path when calling these commands from another directory.
/usr/local/mongodb/bin/mongod
or
/usr/local/mongodb/bin$ ./mongod