Unfortunately, you can't use the hotspot feature on an unlimited plan - on a stock iPad. Sorry, but to use the iPad as a hot spot, you'll have to jail break it. Then, you can use one of several tweaks.
Just know that if you do go this route, your carrier may send you a "cease and desist" notice (for you to stop tethering). If you ignore that, you may end up having your unlimited contract canceled.
Note I said "may" - this is just so you are aware...
Marilyn