Hi Robert,
It looks like the battery replacement procedure on your laptop is one of the most straight-forward ones possible, so I see only two reasons for what you're seeing.
# The battery plug isn't fully inserted into the connector on the motherboard. Unscrew the battery again and verify the plug is all the way in. You can pop the bottom cover back on without resecuring the screws holding the battery to test it.
# There's always the possibility the replacement battery you got is faulty. Although my general impression is that iFixit does a very good job of selecting quality parts to sell, there's always going to be that one bad apple that slips through. They have a very good warranty and replacement department and will be more than happy to help you out with a replacement if that turns out to be the problem.
You might even pop the old battery back in just to verify that everything else is working correctly. If you're sure the new battery was plugged in correctly and the old battery works fine, contact the iFixit Store personnel at support@ifixit.com to arrange for return and/or replacement.
Hopefully it's something simple and you'll be up and running again soon. Let us know how it goes!