I haven't look how the skin you're talking about is coded but in theory the xp bar should stay inside the mainbar if you edit it by code. did you remember to relog after you installed the skin in question ?
And if you have identical entries in the code the ones that appear first in the file gets loaded, meaning you should apply the new code at the beginning of the file.
However you can't break xml heritage, meaning the parent and child tags needs to be in their correct order to get the code working.
ie you can't just add
at the start. you need the whole stuff for that specific part of the code:
you can leave out the child tags you don't need though, but the parent tags need to be there.
but the above examples are just some advanced stuff you don't need to concern about, only if you start editing the code further it helps.
In you case if the xp bar really is way over the left side i'd try to edit the Y value of ID="LevelMeter" and see what happens.
Remember to launch the game again everytime you make changes into SkinDefinition.xml or the changes won't take effect (relogging isn't enough then).