I think the version of tuxpaint we are shipping for Mac from our website (tuxpaint 0.9.21) is failing to start on snow leopard. I've filed a bug on source forge.
I'm very pleased we have http://www.apple.com/downloads/macosx/games/kids_learning/tuxpaint.html 0.9.21 available to download from an official looking place. Yay! How did we get it on there? They also say they've had 0.9.21 since 2008 which confuses me as our 2009 package is 0.9.21.
Our Ubuntu downloads section needs to say how to get tuxpaint for versions other than hardy and gutsy. The only reason we made those packages was because hardy had an old version. They can find tuxpaint in add/remove programs or they can install it by typing sudo apt-get install tuxpaint tuxpaint-stamps. Our page makes it look like tuxpaint doesn't run on Ubuntu post hardy.
Hope this helps