The Apple Support Communities are an international user to user technical support forum. As a man from Mexico, Spanish is my native tongue. I do not speak English very well, however, I do write in English with the aid of the Mac OS X spelling and grammar checks. I also live in a culture perhaps very very different from your own. When offering advice in the ASC, my comments are not meant to be anything more than helpful and certainly not to be taken as insults.
Can you describe what happens when you try to sign into your account with the MAS? What alerts are popping up? Can you provide a screenshot?
I don't know how to do a screen shot but here's what happens:
When I try to open the file there is a grey screen and in the middle it says
Cannot Connect to the App Store.
It won't accept a sign in and efforts produce an error sign in bright red:
Connection Failed. When the failure message appears it's on the sign in menu under my apple id which is there. But my password is not. The same password allowed me in here and allowed me to purchase a Star Trek episode from Iturnes. Weird. Thanks for your help.
You might try this;
1. Got to Sys Prefs/Network
2. Eliminate all of the Network resources by highlighting them and clicking the - symbol at the bottom (be sure to note the configuration before eliminating them)
3. Restart your Mac. Restore the Network resources by clicking the + symbol to add each one and then reconfigure
4. Try the MAS again.
If that doesn't help, I would consider downloading and reinstalling 10.7.5 over the existing 10.7.5 and hope that it corrects what may be missing or corrupted;