In any case, nobody can access that disk at all - I can't figure out how to allow this - so any help would be terrific.
You must have sharing enabled on the computer you wish to access. The disk must be mounted on the computer you wish to access (you should see it in Finder on the computer it is connected to).
On the other computer (the one you are doing access from) you should see an icon in Finder for the computer you have set up with sharing. Click on the icon and click on Connect As, you will be asked to log in (unless you have set up the shared computer with no login password, not a good idea). Log in with a valid user name and password for the shared computer. Depending on your setup in System Preferences > Sharing on the shared computer, you may need to give the admin user name and password for the shared computer to get access to the disk you want. You should then see an icon for the disk on the shared computer.
If that doesn't work or isn't clear, click help in Finder to read about file sharing. Also, there are support forums at Apple.com.