I'm not surprised that Photoshop CS2 isn't running on Snow Leopard. CS2 was created for PPC processors. You're running Photoshop in an Intel Mac under Rosetta emulation which is no longer supported by Apple.
Elements 9 should run fine on this machine. What sort of errors do you get there?
I'm also curious why you bought Elements 9 instead of just upgrading to Photoshop CS5? You're on the cusp of being outside the upgrade window. (Adobe gives a price break if you upgrade within 3 versions back - CS2, CS3, & CS4) Because you own CS2, you can upgrade at the upgrade price ($199 US) - and you can get it for even less right now (amazon and Adobe.com both have promotions going on right now. If you wait for another version to come out, you'd have to pay full price $699 to get the latest version. Just a heads up.
What's your preference? Get Elements 9 running or stay on the Photoshop CS upgrade path.