You go to the part that says Manage Boards in the forum control. Thern go to the catagory that you want to add the child board.... You go there and you and press add board. Then where it says Order, in the first box there is the option of before child of and after.... click on child off and then on the box next to that click the forum that you want a sub-forum for... Instead of being clalled Sub-forum it is called Child-Board... Hope that helps!
-MicroTechXP