Minecraft turns into a black screen with the user clicks play.

I have a Mac OS X 10.5.8 with no app store. I have issued this before but I got no help. One person told me to update my OS Maverick. Since I have no app store, I can't update it. This is how it goes. I log in to Minecraft and make my profile "latest version". Once that is done. I go click play and it shows the Mojang screen. Then it turns into black screen that is fullscreen. I can hear the music but I can't click anything.