I've verified it under Mac OS X 10.8.3 and it works for me for both Juno and Kepler update sites.
Any other clues? Could you tell some more details like:
1. Was clean eclipse downloaded from eclipse.org used?
2. What kind of eclipse distro: standard or Java EE Developer IDE?
3. Did you add any other update sites manually before installing JBossTools?
4. What are JVM version and architecture?