We're gonna need a little more info. 'I plugged it in and it didn't work' really doesn't help much in diagnosing the problem. Have you gone through the Getting Started Guide? There are a number of steps involved, particularly making sure that the correct physical inputs are routed to the correct channels in the software (Devices --> VST inputs; also, your mixer channels can be used to select inputs) and that the software channels are activated for recording (the button with the black dot in the middle that turns red when you click it).
Also, and I'm not accusing you of anything, the fact that you are using a $1500+ piece of software (Nuendo) with a $200 interface (Tascam) is suspicious. In general, people on this board (myself included) do not tolerate software piracy.