Verify that your carrier supports Personal Hotspot for your device and that you have Personal Hotspot enabled on your cellular plan:
From Here > http://support.apple.com/kb/HT3574
Personal Hot spot
iOS System Requirements > http://support.apple.com/kb/HT3574
Understanding > http://support.apple.com/kb/HT4517
Trouble Shooting > http://support.apple.com/kb/TS2756