Solution
In simulation mode, the only clocks available as reference clocks are the FPGA high level clock and clocks derived from it. When running in simulation mode, only certain IOs from the FPGA are acessible and the IOs of the CLIP of the AT-1212 module are not part of them.
If your clock IoModClipClock0 is running at 156 Mhz, the solution is to
create a derived clock at the same frequency. You will then be able to select it as the Reference clock in the scrolling menu.