I just got my Macs a month or so ago, so I can't walk you through anything. But I do this all the time, without necessarily knowing what I'm doing.
Create a network with a password on your iMac. Automatic channel works best for me. Connect to your network from the other machine, using your network password.
On both machines, select Go from the top menubar-->Network. Click "Connect as..." and log in with your actual name and account password for the other machine's account--not your network password.
Go to System Pref-->Sharing and make sure you're allowing file sharing.
You should be all set, I think. Can't remember what else I did in setup to make it work.
It is, however, way too complicated for something every owner of 2 macs is going to want to do. Ought to be a wizard or widget to remind you not to turn off sharing, etc.
Remember to unmount anything you mount before you're done.
Good luck.
P.S. Oh, these instructions give you access to everything on the machines. If you just want to use the drop box you don't need to log in to your actual actual account remotely.
Message was edited by: Marakel