There's a couple of choices there... you can go to the Bliz settings menu by pressing ESCAPE and then turn on combat text. There are also quite a few mods that are exceptional at this ability: SCT being one.
At this time, nUI does not have this ability enabled though a good bit of it is already coded, I'm having issues with taint with it and can't release it until I fix that.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.