@googlefanz -- You definitely need to get rid of any stains on your concrete before you even think about epoxy floors.
If they're just oil stains, then you can try to scrub them out with a metal-bristled brush. However, like the article said, make sure that your floor is completely, 100% dry before you start to put down your primer and paint.
Although it may be tempting to just paint over the stains, don't.
The uneven surface and the difference in the smoothness between the stain and the rest of your concrete will start to show, and pretty soon your pretty new painted concrete floors will be in tatters.