I tested Marked Enemy a little and, if I'm correct, this skill works only for ranger and adds 20% damage bonus, but calculated after taking into account DR. So if you'd normally hit enemy for 15 - 7DR = 8DMG without marked enemy with marked enemy you will hit him for (15 - 7DR)*1,2 = 9,6 (I don't remember how the game rounds it).
Additionally it turned out for me, that if you deal less damage than enemy's DR you will deal some damage, because it seems you always deal some damage, but you won't get anything from Marked Enemy - kinda logical since 5DMG - 10DR < 0, gets rounded up to 0, 0*1,2=0, but since hit in PoE means you always deal some damage you deal a little.
It's a very disappointing skill, if it worked as it is written in tooltip it could decimate single enemy quickly, but practically all fights involve more than one enemy, so I don't think it would be too OP.
But maybe it's better that descriptions are off? In the current state my use of ranger is limited to ordering him (well, her) to shoot something, I don't even bother with trying to keep the same target with pet since it usually is there only to tank 3 hits before dying. Now it may appear good in description at least, so somebody may be lured into playing as one thinking he may be good.