It's been a while since I've uploaded issues via the XML import path, but looking at a sample XML file there is the issue-level tag:
<issue published="false" current="false">
To have an issue be published, and then to be listed as the "current" issue without any extra measures, you'd switch those two flags to true. To upload issues as published but not current, you would set the flags as true and false, respectively.
Otherwise, you should be able to check your unpublished issues from User Home --> Editor --> Future Issues. Click on an issue title, and click the "Publish Issue" button that appears there. I think the issue with the most recent published date will automatically be listed under "Current Issue", but just to be on the safe side, publish that one last.