I have a strange issue with importing catalog data.
When I export a xml file from the commerce's manager and I try to import it into other commerce site the "start import" button does not work. It is in disabled mode.
It could be a bit tricky to figure out :)
When you upload sleected file it should appear in "Files Available For Import:" list. You have to select particular file to enable "Start Import" button.
Try some other browser if you are using IE8/9