Customer Coupon

I had using one excellent extension which enable to specific the customer for coupon discount.
extension link:http://www.opencart.com/index.php?route ... r%20coupon
However, some of my error appeared that the customer group is not allow to choose as I cannot select the customer group due to nothing pop out after i click or type the customer group name on the input field.

Therefore, we are looking for someone who able to diagnostic and solve our problem. Appreciate for all Opencart member can guide me.