Welcome to the forums! When you say it didn't fit what do you mean? The rack wouldn't fit in the receiver, if you have one, or is it a trunk mount? What brand and model is it? What year is your car? I know this is not an introductions section discussion and will be moved to the proper place when a mod sees it, but give us some more info so we can better assist you in finding something you can work with.
Car: 2010 Subaru Impreza (Outback Sport)
Mount Thule 599XTR http://www.amazon.com/gp/product/B0001HACDM
The reviews said it fit Subaru, so I bought it. Apparently I should've paid more attention, as it said it fits more "square and round bars". Mine are sort of flattened ovals.
According to the Thule Fit Guide US vehicle guide for roof mounted bike racks using the factory installed cross bars for the 2010 it looks like they suggest: 513 Domestique, 594XT Sidearm, 516 Prologue, 599XTR Big Mouth, or 598 Criterium. I am not sure if anything else is needed for those carriers to work, but at least you can see what the company recommends. Hope that helps you a little bit.