I'm not sure what you mean by "Auto Activate" a new member? There is no "Auto Activate" option. It would not be a good idea to have one, because of the "SPAMMERs" and other individuals who would misuse your forum.
To activate a new member’s account the member should click the link provided in the “Confirmation Email” sent to them. However, for you as an “Admin” to activate the account for the member (if the member does not follow through with clicking the confirmation email link provided) you need to log in to your “Admin Options”; click on “Members Pending” (this is located on the far right, top item); follow through with activating the new member(s) that you wish to have join your forum.
Also you might take a look at these topics for info on other member’s confirmation email problems?