Updated to Yosemite, now on 10.10.1 and the external display disappeared. It’s running through a targus dockingstation displaylink.
There is multiple post about the issue on dipslayLink forum, some users got it to work: http://www.displaylink.org/forum/forumdisplay.php?f=30/
Tried both running the lastest DisplayLink 2.2 and the 2.3Beta, none of them work for MacBook Air 2013. Ended trying installing an old release 2.1 and this apparently work, even though it’s stated that it works up until 10.9 Maverick. But not concerned about it, you can download the drivers from here: http://displaylink.com/support/mac_downloads.php