But it says right here »www.optimum.net/ExploreOptimum/f ··· 56424542
that the Kindle Fire HD 8.9" and Kindle Fire HD 8.9" 4G are not currently supported.
Did you not check device compatibility before you purchased the device?
It also says that they are working hard on getting those devices supported -- so maybe you'll be in luck soon. You could switch to the 7" Kindle if you need instantaneous gratification.