That's probably because of a small typo, the file should be called SkinDefinition.xml
- there's an "i" missing from your file name
the tga file can be in the same folder as skindefinition.xml, it really doesn't matter where you put it but you need to have the relative path info correctly set in the code then.
If you have <Mapping ArtAssetID="player_vitals_backdrop" FileName="Graphics\vitals\opponent_vitals_backdrop_master_d efender_good_mp.tga"
then you need to make a subfolder Graphics
and another beneath that one called vitals
and place the tga file there.
If you have <Mapping ArtAssetID="player_vitals_backdrop" FileName="opponent_vitals_backdrop_master_defender_good_mp. tga"
then you can place the tga in the same folder where your skindefinition.xml is.
In addition to that you might need to place your files under \UI\Skins\ not just \UI\. It might work from UI folder only, I haven't tested for a while but in the past it didn't.
But first fix that file name and if it won't work then try those additional tips.