Several possibilities. You may have MAC filtering enabled on the MiFi. You may have entered the WPA key incorrectly on the iPad. You may need to update the firmware on the MiFi. The current version is 1.59.
On the iPad go to Settings/WiFi, tap the arrow to the right of the MiFi name and then "Forget this network". Then try again. If it still fails go to Settings/General/Reset and tap "Reset Network Settings", then try again.