Try placing the files into sdcard\Media\Audio\MP3 .. I have never had a problem not seeing any MP3s I've put in this location. That is where the phone looks for all ringtones, so I figured it was the best place for my music.
If that doesn't work either, your phone might not be rescanning the sdcard. Have you mounted over usb and disabled it so it scanned? I know there's an app out to force a rescan without all the extra steps, but I can't remember the name of it.
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