I put its folder in the Trillian "skins" folder, it isn't selectable from the skins selector.

Make sure you don't have it in a folder within a folder. I've done that before after unzipping the file. Other than that it should be selectable.
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Russ
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"The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack of will." Vince Lombardi