So maybe you're like me and bought a new Airport Extreme Base Station (802.11n) and want to extend your wireless network through an older Airport Express (802.11 b/g). After Googling this, I discovered I wasn't the only frustrated user. Apple wants you to buy a newer Airport Express with 802.11n to make extending your network easier. However, you can still use the older Airport Express with a little work and a hidden trick.
For some reason, this took me SEVERAL hours to figure out. It didn't JUST WORK. LOL! Fortunately, I learned that using the Option key while selecting the Wireless Mode popup menu in the Airport Utility (Version 5.5 (550.29)) returns to the user the ability to set up a WDS (wireless distribution system).
A nice user named Bob on the Apple Discussion forums confirmed the Option trick I had learned elsewhere. However, I still had trouble getting the WDS to work properly. Here's what I tried that seems to do the trick:
Thanks Bob. Yes, I found that trick elsewhere. Still had trouble, even though I went through all the proper motions (i.e., making the AEBS(n) the WDS base station and the APX the WDS remote...including entering the correct MAC addresses for each).
However, I think I've discovered the fix that allowed my old APX [Airport Express] to work with the new AEBS [Airport Extreme Base Station] in a WDS network: I set the radio mode on the APX to 802.11g ONLY! The radio mode on the base station was kept at AUTOMATIC. And I used Channel 1 for the 2.4 GHz frequency on both and left the 5 GHz frequency channel selection at AUTOMATIC on the base station. WDS wasn't working until I made those changes (don't know if one or more of those steps is important...but it's working now).[From Apple - Support - Discussions - WDS set up problem ...]