As an registered member you should see a "Submit
" on the right side menu thing on the main page. From there you can select an upload category. Select the one best suited for your upload.
You can see the category descriptions at the download section
(in your case I suspect you should select LotRO Compilations)
After you have done that you're taken to a form where you can add your file (compress your files with ZIP), edit the description and add some screenshots. You can author this page later on at any time after your file is approved by the moderators and your skin/plugin download page is created.