Connecting via Target Mode as rab suggests will allow you to access the hard drive on the eMac via the MacBook. Unfortunately if you install a system this way it will be a system for the Intel processor on the MacBook Pro and will not run on the eMac. You need another Power PC processor machine. Connect them via a Firewire cable and start the eMac holding down the "T" key. Then use the optical drive in the other machine to install the system.