Not Letting Me Purchase Games

So, I recently added 30 dollars into my steam wallet, and I wanted to buy the new game "Verdun" and the Black Ops Zombies DLC, "Rezurrection". I add the item to my cart, yet when I click "purchase for myself", It gives me a little message at the top of the page stating "there seems to have been an error initializing or updating your transaction. Please wait a minute and try again or contact support for assistance". I am a little frustrated over this and have repeatedly checked the steam user forums and basically every other forum website involving steam until I reached the bottom of my list, which I don't know why wasn't at the top, Harpoon Gaming Forums. I own a Mac which I play my games on (I know it's a crappy computer, no need to comment), and I would like to know if one of you techy guys out there have a solution for me. I need specific Mac instructions, and before you say it, I have already filed a support ticket. I am very frustrated at the Steam Support because the steam sale ends tomorrow. I would appreciate it if one of you could give me some instructions, so I can buy the game I want, without adding another 15 bucks into my wallet.

Thanks

 

Here is a picture of what happened so that you can get an idea