After creating a attack you must import the attack by opening the Attack List editor on the Actions>Attack List tab. Once in the editor you can import the attack and once you close and reopen the editor the attack should show up.
In the event that the attack does not show up for some reason, you should close the Avalanche GUI, delete the file called "threat_cache" from the C:\Program Files\Spirent Communications\Spirent TestCenter [version number]\Layer 4-7 Application\database\threatex\ThreatDB directory. After doing that when you open the Avalanche GUI and go into the editor it will rebuild the attack list and the threat will show up.