I haven't done the iMac Pro, but I've done quite a few standard iMac's that have the same stand design. There is a small latch that you need to disengage using the tool that came in the kit or a credit card. Review this Apple manual for the [https://manuals.info.apple.com/MANUALS/1000/MA1823/en_US/iMacPro2017VESAMountAdapter.pdf|iMac Pro VESA Mount] to see where you need to slide it in.