Trouble with UnitHealth("player")

[Thalas]

Hi, i'm new to Addons and before doing something useful, i'm practicing on making interfaces and stuff, and I found problem when I tried to 'write' the player's health on the addon (or any other data from him). I couldn't find a way to make it go on the Frame as a text, if I try to mention it on the XML file, the addon won't work, only on the LUA file. No problem with that, as long as I can make the XML file get the required info from the LUA code, but I can't seem to find a way to do so.
If anyone could help with that, i'd be really grateful. Thanks in advance and sorry for any English mistake.

(And sorry if this is not the right place for this, I didnt know wheter put it on XML or LUA forum, so feel free to move it please).