I had to format my computer a few weeks ago and redo all my ui. I use pitbull 4, I can't see incoming dmg on myself, target, target's target, focus etc. It's a big deal for me since I'm a rogue, I need to know when a dot ticks on me so I can re-stealth at the right time and reopen without failing.
I looked everywhere and I just cant find where I can make it tracks it. I was using pitbull4 before my computer crashed and I could track it.
Are you just talking about combat text on the unit frame? If so, you can enable it (and move it) under layout for each frame. In my experience though, this isn't the most efficient way to track incoming damage.
Another thing that I didn't mention is, make sure that module is enabled on your addon list in the character screen.