There could be a few reasons why your affiliate isn't appearing in the drop-down menu.
To get it fixed, talk to your gym owner. Let them know their affiliate is not on the drop-down menu, and that they can get it changed by contacting Affiliate Support (firstname.lastname@example.org).
In the meantime, continue registration. Click "My CrossFit affiliate is not on the list," enter your affiliate's name and select the country in which you reside.
You will be able to add your affiliate later through your profile page, once the gym owner and affiliate support get it all squared away.