Unfortunately, about 4 years ago when Apple introduced the 6.x version of AirPort Utility......the application used to configure and adjust settings on the Apple AirPort routers.....they removed a number of settings that they felt that users no longer needed. Adjusting location was one of them.
As Meg notes, the AirPort Extreme will eventually figure out where it is, but it might be 4-6 weeks or longer before this occurs. In some cases, it never seems to occur.
We've seen a few posts from users who claim that resetting the AirPort back to factory default settings and then setting it up again has corrected the location status, but this has never worked for me when I've tried it. However, you may have better luck......and this is really the only thing that you can do that might help things along unless you have a Mac that is running an older 5.x version of AirPort Utility.