Launch /Apps/Utils/AirPort Utility and select each base station, one at a time, and the window that opens will tell you that you need to update firmware (if, in fact, you do need to update firmware).
If you read keys initial post and looked at the image it linked to, it indicated an Airport Extreme that needed one update and the Airport Extreme is in turn connected to an Airport Express that needed one update. The popup is an automatic checking function built into later versions of Airport Utility. I don't know if these versions are compatible with the older versions of OS X that you are using or not.
