1. Make sure your "Connection" on MobileHotspot is set to your mobile provider.
2. disable 3G internet connection before starting MH; MH will create the connection as it launches.
3. if the above doesn't work, enable 3G internet connection before starting MH.
4. if the above doesn't work, reboot, try again.

MH is a bit finicky to say the least, and depending on which carrier you have, and your location, connection can be spotty. Rebooting usually solves MH problems.