If you have a newer dual band AirPort Extreme, one of the problems with trying to extend using an AirPort Express is that it can only extend one band from the AirPort Extreme.
We need to figure out whether the Express is extending the 2.4 GHz band or 5 GHz band.
If the MacBook Pro is connecting to the Express as you state, hold down the option key while you click on the fan shaped AirPort icon at the top of the screen and look for the Channel info. This willl indicate whether the Express is extending the 2.4 GHz or 5 GHz band.
Then, do the same option-click check with your iMac and see what band it is connecting to.
Post back on your results.
Thanks, Bob. The laptop connected to the Express is getting 5 GHz. The iMac connected to the Extreme is getting 2.4 GHz. I managed to get the iMac to connect to the Express by restarting, and got 5 GHz. But now it has reverted back to the Extreme (2.4 GHz).
While I was sitting here typing this, I noticed AirPort Utility acting like it was scanning the devices again. My iMac is now connected to the AirPort Express (5 GHz). I would love it if I could force it to always connect to the Express. I get a much more reliable (and way faster) Internet connection through it. Transferring files from machine to machine works much better this way as well.
The Express is only a couple of weeks old. I got the Extreme in April of 2010.
You might want to try assigning a separate name to the 5 GHz network on the AirPort Extreme, and then re-configuring the Express to specifically "extend" that 5 GHz network name.
If you are not sure how to do this, post back to let me know which version of AirPort Utility.....6.0 or 5.6....that you are using.
Once you have verified that the Express is extending the 5 GHz network name, connect up to that network by "pointing" your iMac to that 5 GHz network name.
Then on the iMac....
Open System Preferences (gear icon on the dock)
Open Network, and click on Wi-Fi
Click Advanced at the lower right of the window
Make sure that the name of the 5 GHz network is first in line at the top of the connection list so the iMac will connect to that network first. If the 5 GHz network is not at the top of the list, drag it there.
Click Done, then click Apply
Try things out.
Using AirPort Utility 5.6........
Select the AirPort Extreme and click Manual Setup
Click the Wireless tab below the row of icons
Click the Wireless Network Options button near the bottom of the window
Click Done and then click Update to save the settings and let the AirPort Extreme restart
Enter a check mark next to 5 GHz Name
This will automatically add "5GHz" to the name of the 5 GHz band network
Click Done, the click Update and allow the AirPort Exreme to restart
On the AirPort Express....
Open AirPort Utility > Manual Setup
Click the Wireless tab
Make sure that the name of the network that want to extend = your network name 5 GHz. This tells the Express to extend the specific 5 GHz network that you named on the AirPort Extreme.
Click Update to save the settings on the AirPort Express.