Starting from version 3.0 the tracker works only with the binary geo library from MaxMind (http://www.maxmind.com/app/ip-location). You need to put one of these files in the 'store' folder in the tracker's directory: GeoIP.dat (only countries) or GeoCity.dat (countries and cities). Please note that you should keep the exact name of the file.
If you are adding the geo library to the tracker for the first time or updating it with a new version, you also need to click on the 'Update database' button after the new files have been added.
You can also use a free version of the MaxMind's cities geo library called GeoLite (http://www.maxmind.com/app/geolitecity). It contains almost the same information as the commercial version, but the data is less accurate.