The Zwift Support hub indicates zPower support for a limited number of classic (i.e. non-smart) trainers, but I couldn’t find instructions for identifying which of these classic trainers I am using. I finally figured it out this morning. Hope this helps others in the Zwift Community.
After logging in, Zwift presents the “Paired Devices” dialog showing Power, Speed, Cadence, and Heart Rate sensor pairings. After pairing your Speed sensor, press “Change” within the box for the Speed sensor. Zwift then presents the “Searching” dialog box. Select your Speed sensor (it is already selected if only one Speed sensor is found) and then press “OK”. Zwift then presents the “Select Trainer” dialog. From this dialog, you can select either the “CycleOps Fluid 2”, “Kurt Kinetic”, or “Travel Trac Comp Fluid” zPower supported trainers, or “Not Listed” if you’re using a trainer that is not supported with zPower. Others may be supported at a later date, but these are the options as of Feb 2, 2015. Select the desired trainer and then press “OK”. Your selected trainer will then be displayed on the “Paired Devices” dialog within the box for the Speed sensor.
It appears that you will only have to do this once (i.e. you will not have to repeat the process each time you log in).