Grace,
While LR will read metadata from XMP sidecars for raw files, it won't do so for JPEGs or TIFFs. For those, you have to select the Aperture option to write the data directly into the exported masters. See this article:
http://lightroomsolutions.com/articles/migrating-from-aperture-to-ligh troom-where-do-i-begin/
(Look for the comment by Rick Hagen.) The author, John Beardsworth, hangs out here, so perhaps he can chime in with more details. (I'm not an Aperture user.)