In case it's of use to anyone, here's what I had to do to get my Virus Ti working in plug-in mode on my new Mac Pro (2013 cylinder).
My problems seem to have been due to multiple causes.
1) Make sure you're using a USB cable like the one supplied with the Virus - usually a braided see-through plastic covered cable. Standard USB cables worked for my on my 2011 MacBook, but the Mac Pro / USB3 system is more fussy.
2) Boot the Virus fully before powering up the Mac, otherwise Virus shows activity on the plug-in, but DAW (Cubase and Ableton in my case) get no audio returned via USB. When powering down, fully power off the Virus and the Mac.
3) Use a Thunderbolt to USB3 adapter specifically dedicated to Virus. I've got the Sonnet USB3 / eSATA adapter. I also have one Thunderbolt channel dedicated to the Sonnet adapter.
4) 126.96.36.199 beta drivers seem to be OK for me, but the now removed 188.8.131.52 beta also seemed OK.