The modem is not designed to work directly with a Linux host, although Linux in general certainly can be used with 3G modems. The Tivo, however, is not designed to work with the modem, either. You are going to need a router. If you are unable or unwilling to build your own, then you will need to buy one. Zoom makes a nice, economical 3G router, the 4501. They also make a travel router, the 4506, and they make a mobile hotspot / modem combination that might work with your service. The modem compatibility list for the 4501 and 4506 is here
. I do not see "Optus" on the list, but chances are the modem is not made by the service provider. You should be able to find a manufacturer and model number on the modem, and the chances are it will be on that list. It may even work if it is not on the list.
Assuming you are running Windows and your version is XP Pro or W7, hypothetically, you could set up your PC as a router / internet gateway by activating Microsoft's Internet Connection Sharing protocol, but I suggest you take my advice and don't bother.