Catrike 700, Greenspeed GTO trike, , Linear LWB recumbent, Haluzak Horizon SWB recumbent, Balance 450 MTB, Cannondale SM800 Beast of the East
The computer itself mounts on the part of the frame where you see the front derailleur (stub sticking out to the left). You can mount both a light and a computer on the same piece. That's how it worked on my Catrike 700.
It should have come with a computer mount for the sensor already in place by the left wheel. That is standard equipment with the CT 700 I bought in March 2013 and should not have changed. Here is a link to what it looks like: Database Login page. It is the second item down on the list. It mounts on the front left, right next to the left disk brake.
There are a couple options to mount the computer itself. In the past I was riding a Catrike Trail and built a bracket that was kind of 'A' shaped attached to the water bottle bosses in front of the seat. On the front of the 'A' I installed the water bottle cage so it would stand up and not drip (easier to access too). On the top of the A, I mounted the computer where I could see it (closer than the derailleur stub at the crank). That worked pretty well. On my new V81 Annihilator, there is an accessory stem on the left handlebar that holds a mirror up by the left front wheel. The computer is also mounted on that stem. Lots of good ways to mount accessories on a trike, if you are willing to fabricate or purchase an additional piece.