All of the discussions with pre orderers happen in the SPF, which you can access by following the steps on your humble bundle page like in this thread: viewtopic.php?f=2&t=15892#p170453
However, I can tell you that the issue you are having is a compatibility issue with 10.5.8 and Overgrowth. We think it might be related to Awesomium, which OG uses for it's UI, but I'm not sure if/when that will be fixed.
Updating to 10.6 or newer will solve that problem, or you can always contact the devs at [email protected]
and they can grant a full refund if it simply won't work for you.