Basic structure for Skindefinition.xml would be:
<opt> needed to start the file
<SkinName Name="SKINNAME"> gives the #NAME# for the skin
</SkinName> ends the naming tag
<Panelfile> includes numeric values for panels
<Element ID="PANELNAME"> child element for panelfile-tag
</Element> ends element-tag
</Panelfile> ends panelfile-tag
<Mapping ArtAssetID="PIC.TGA"> refers to a picture used for skin
</Mapping> ends mapping-tag
</opt> ends the file
If you're missing even a single </Element> tag that should be there in the correct location the skin won't show up in Lotro.
The problem might be a much more simpler too, hard to say.