Close down Opera Mail and delete its "mail" folder (see help -> about Opera Mail for the location). When you start Opera Mail back up, that import option you want should reappear. Before doing anything else, try it again and be really, really patient.
The other way to do it is to replace Opera Mail's mail folder with the mail folder from Opera 12.x. In addition, you'll want to copy contacts.adr, opcert6.dat, operaprefs.ini and wand.dat from Opera 12.x over to Opera Mail (replacing existing Opera Mail copies). They're in the preferences folder. See Help -> About Opera in both Opera 12.x and Opera Mail. Then, before you open up Opera Mail again, edit Opera Mail's new operaprefs.ini (that you copied over from Opera 12.x) and remove any lines that have a path in them so that Opera uses the default location for things. If you use a custom mail folder location though, make sure to keep the Mail Root Directory line.