Any programs like ISIS Draw or Autonom for Mac OS X

Hi, I am a student and its frustrating that I cant seem to get any of these programs. These programs draw molecular formulas out for my Pharmacy Course.
Ppl with pcs laptops etc are able to and it puts me in a great disadvantage. And there are other things too... Mac aint good at compatibility...
ISIS Draw and Autonom works for older versions of mac i.e Mac OS but doesnt work on X.
Can anyone post a freeware drawing program you experienced that is good and easy installation... thanks.