Ah. I didn't know you were calling it with runSnippet(). Have you tried just putting a FileUpload tag on a page? That might tell us something.
It looks like, for some reason, it's not loading the lexicon file, though I can't see why.
Make sure the lexicon file is there at core/components/fileupload/lexicon/en/default.inc.php and that it contains the lexicon strings you're seeing.
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODX info for everyone:
http://bobsguides.com/modx.html