Did you purchase an RV-12 avionics kit from Van's Aircraft?
If you did, then the trim switch is on the left side of the switch panel at the center of the instrument panel as shown HERE
If you did not, then you need to design it yourself using the appropriate switch for your installation.
A basic Skyview system has an indicator show pitch trim position
, but it does not actually control it.
If you install the optional autopilot panel, it does provide for automatic adjustment of pitch trim by the autopilot, but I think Dynon wants you to still have an alternate switch for setting trim to take-off position, etc.