Walking Dead Major Mac Bug

I have been frustrated greatly with the game I purchased on Steam called The Walking Dead. The description states that I will get all five episodes for the cost of $25, but as I purchased and played through it, I could only complete 3. When I began episode 4, the screen simply went back. I have an Intel-Based iMac running Mountain Lion, and my computer is in great shape therefore this is not the issue. I have played through episodes 1,2, and 3 twice but it will not load four. Now, no matter what game I load it always loads "Next Stop, The Atlantic" even if I start a new game or load a previous one. This is a huge issue and I would like the $5 I paid for episode four and the $5 I paid for episode 5 if I will not be able to complete it. I have tried many fixes but really, the game should be patched. Are there any other fixes? No matter what I can't play episode 4.
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