i) Make sure the email address given in AB is the same as the one used to register with Gravatar. If not, change the AB one to be the same.
2) Click on the box to activate your avatar in your AB Profile. and then click 'Submit'
NB. In some browsers, the box to tick is not visible, but it is there anyway. It is just below the 'Your' of 'Your avatar is currently inactive'. You will know when you have moved your mouse to the right location because the icon will change. Have faith and click it.
Take care not to click twice because it toggles between on and off.