Right click on the "Quake3: Arena" tab in the left panel. Select "Add Master". Fill in these fields:
- Master Name = Urban Terror
- Leave drop down box on "Quake3: Arena"
- Master address = master://master.urbanterror.net:27950
- Keep the radio button on "Standard"
Now click on the new Urban Terror tab in the left panel. Click the update button. Currently, it shows 117 servers for me. Update will ask the master server for new addresses and then refresh the servers. Most of the time I just use refresh because the server list is generally static.
If you don't do this and instead use the general Quake3 item in the left menu, there are 1,725 servers. As long as you only refresh and update the urban terror menu item, you should get refresh times of about 5 seconds.