ZTE V9 issues - SIM Card

[Bailey]

I have a second hand optus tablet ZTE V9 (which was working and is unlocked) but after being "factory reset" the device won't regonise the Optus pre-paid sim card.

The device is running Android version 2.3.4.

I have gone into settings / Wireless & network settings ... there is no VPN settings currently set up and the "Mobile network" is shaded out (can't be selected).

Please help ... I have no idea on what to do (please give simple step by step directions )