you do have the tick box, it's just invisible (seriously!) if you click once where the tick box should be visible and then click submit it will work (as long as you actually hit the tick box when you clicked)
if like me, you see neither the check box, nor the text beside it, just remember it is located just below the "Yo" of "Your avatar is currently inactive.".
Just click it once (it will toggle back to off again if you double-click it)
You will know when you are over the check-box in IE because the cursor icon changes from an I-beam to an arrow when you are over it.
Once you have clicked it, scroll down to the bottom and click 'Submit'
as long as you have used exactly the same email address on gravatar as you have on your profile here on AB then it should automatically show your avatar in a few minutes.
Thanks for this CF. I've got my account with Gavatar and the pic is there but nothing is happening on here. Do I have to type in AB's address on the Gavatar site to link it up?