The one referred to in the last post is found at www.dropbox.com
Register for a free account, install it and configure it on the MacBook Air
Download it on the iMac too.
Now you have the Dropbox installed on two machines, using the one free account.
Anything you put into the Dropbox folder on the Air, will be seen in the dropbox folder on the iMac.
Go from there.
Regarding Logging in from the iMac. If you use "Connect As" (a button seen at the top right of the window that shows when you have selected the computer you want to log into) and use the username and password of the owner of that computer, the entire computer will be available to you. Select the HD and it will mount on the desktop. The Drop Box you can see does not allow you to see inside it. If you can see the Drop Box of the iMac from the Air, just dump the files you want to move, into it. I think the message is that "you cannot see the files you drop in here" (or words to that effect.) That is normal. The owner of the Drop Box, (iMac) can retrieve files from it.
The point of using Connect As, is that you do not have to mess with the Drop Box, you have the complete run of the machine.
P