Are both of your meters reading? You have to LED-lines on the right, they're for your headphones. Let's do this step by step.
First, plug you bass in, let's say in the upper left jack. Switch your "buss"-switch to, yes, "buss". If you play your bass now you should see three signals, two on the right for your headphone, and one on the left indicating that you're playing on track 1. If this isn't the case, make sure that your tape/mic-knob (the first knob on the upper left) is set to "mic", to the right. Make shure your volume-control fader and your master fader are set to the shaded area (around 7 or 8). Be careful if you have your headphone-control all the way open since you'd blast your ears off if this would work!
If all this isn't going to work, then start all over again according to the manual. Make shure all jogs, knobs and faders are set according to the manual. If it's still not working something's wrong with it.
The TASCAM is a great buy, (I have a mkII), it's a small machine with lots of features, and that also makes it difficult sometimes. When you get the hang of it (and treat it right) you'll never want anything else. For now. If you have any questions I'd be happy to help you out!
Dirk Demon