The response of http://marketplace.eclipse.org/catalogs/api/p is probably also your answer.
I'm having the same problem in Windows 7. And since i couldn't find anything on Google either i'm guessing this is a temporary problem caused by some bug at the marketplace.eclipse.org server. I've tried installing Kepler for Java programmers and the Standard edition, both return the same error when trying to access the marketplace.
Good luck.