1. Open ~/Library/Application\ Support/Tunnelblick/Configurations/ with Finder.
2. Rename openvpn.conf to openvpn-location-a.conf.
3. Duplicate openvpn-location-a.conf and rename the duplicate to openvpn-location-b.conf.
4. Open both of these configurations and change values such as server IP, etc.
5. Restart Tunnelblick.
This should be it. Now you should be able to choose between configurations when clicking Tunnelblick’s tray icon.