Should become one automatically if you:

1. Have fast internet. (at least 512 Kbps/about 60 KB/sec up) (Only allow 48 KB/sec of them as Ultrapeer mode will use up to 10 KB/sec for messages

2. Are properly configured. (no firewall icon)

3. Have enough uptime (I heard somewhere at least 6 hours on each time you run Limewire)

4. If the network actually needs UltraPeers at the moment

(it was 32 BTW)