Connects from A to B, but not from B to A

I have an AirPort Extreme with an iMac connected via ethernet and a MacBook connected wirelessly. The iMac is running OSX 10.6.8; the MacBook 10.5.8. Both computers have Airport Utility 5.6.1. I also have a printer connected wirelessly via the Extreme.

From the iMac I can connect to the MacBook and read and write from/to the MacBook. No problem. But from the MacBook I cannot connect to the iMac. Here's what I do from the MacBook:

- Click on iMac's entry in Finder's left-most column. Next column then acknowledges the iMac, says "Connected as: (me)" with a "Disconnect" button underneath it.

- Click on the "Disconnect" button. "Connected as" message changes to "Connection Failed" and button now says "Connect as ..."

- Click on "Connect as ..." button and get a pop-up requesting my name and password.

- After providing my name and password, Finder's second column reverts to "Connected as:" with a disconnect button, just as before.

This cycle will go on forever.

And just to keep things from being too simple, BOTH computers can print to the wireless printer via the Extreme.

My guess is that I need to make some kind of setup adjustment somewhere; but where and what?

Any suggestions greatly appreciated. TIA -

TheOldFart