I feel horrible for making out with a good friend's ex and coming clean to the friend
What made you feel so horrible?
I feel guilty for my actions.
I understand why you are feeling bad for your actions, but coming clean was a good thing.  You should not have betrayed you friend, but being honest is the best first step toward fixing the problem.
I don't understand why the friend isn't talking with me. Should I apologize?
Yes, if you told your friend what had happened without apologizing, he might have thought that you did it deliberately, which is why he is so mad.  Apologize right away, and explain that you meant to express regret all along.  Your friend might still be mad, but that is a necessary first step.