can't open minecraft

"affects version" above is useless because I don't know which version of java i am downloading. 

 

download minecraft. install minecraft. try to open minecraft. screen starts flashing with a prompt in the background that says updating minecraft, but doesn't do anything. have to restart computer, delete minecraft, reinstall minecraft, and the same thing happens over and over. 

We paid $28 for this game and we can't get it to work on our iMac.