Hail, as a returning warrior from a multi year hiatus, I am unable to identify where the perk slots are located on the 3-21 configuration. Regardless where I place the square perk icons, I receive a message indicating that it is not activated. I depressed the top silver area in the top middle of the 3-21 configuration, but the alternate slots on the top left that appear will not accept the square perk icon. The perk icon is in color and highlighted on the AA listing. Thank you in advance for the clarification.