Feel for a spring-latch inside the gap, about ¾ of an inch the way in. If your card goes in more than that, you’ll need to remove your card and try again. When you hear a quiet snap, you can continue. [1] X Research source
You can buy one from the Apple Store or one should have come in the VESA kit you bought. If you don’t have access to either of those sources, you can easily find one for cheap from online retailers.
To re-attach the stand to your iMac, follow these instructions from the end to the beginning.