cant upgrade to snow leopard

i recently purchased Mac OSX snow leopard because i was told that my Mac Osx 10.2.8 can not upload my yahoo messenger,,,,but when i loan snow leaopard it says the system is incompatible....what do i do,,,,i need to get yahoo messenger and i do i return snow leopard and what can i do so my computer will download and open yahoo meesenger? thanks so much, miss cheyenne foxe