Go to the Sharing pane of System Preferences on the iMac. Name the iMac if it does not have a name or rename it so its something you can immediately identify. Don't rename if you backup with Time Machine. Enable file sharing, and then add to the list of shared folders the directory that you want to have accessible from the MacBook Pro. Restart the iMac, then look in the Finder sidebar on the MacBook Pro for the name of the shared iMac. Connect as a registered user instead of as a guest, and drag whatever files you need to the Desktop on the MacBook Pro.