Can you post a screen shot please?
(ps: nUI doesn't "use a unit frame" -- it *is* a unit frame)
When you load nUI... it should look like this...
http://s.wowinterface.com/preview/pvw24241.jpg
If you look at the unit frame for "Kitty" -- my hunter pet -- you'll see Kitty's current and maximum health in the top right the unit frame... current health is 9976 and max health is 10824 -- Kitty's percent health and percent power (focus in this case) is immediately to the right of the health and power bars... 92.2% and 14.0% respectively.
If you look at the party example at
http://s.wowinterface.com/preview/pvw24252.jpg you can see my unit frame (Erranthor) left of the minimap. It, too, shows current health as 15974 which is 100% (shown on the left side of the health bar) while my current power (mana) is at 89.0% (shown on the right side of the bars) -- likewise, if you look at the party member Sessnishi you can see his current health is 23300 or 99.6% -- my target's current health is 385 or 2.4%
The information is all there even without the HUD.