To multitrack in Wavelab 5, you have to use the ASIO drivers. Start my going to Options>Preferences and click on the "Audio Card" tab. Using the "Playback" drop-down, select the ASIO driver for the device you are using. The "Recording" section will gray out. Here's what it looks like on my tower with a M-Audio FW 1814:
Then click on the "Connections" button and you will see channel mapping options, based upon what you have available:
Here you can choose which inputs you want to use for your device. Once set your inputs, click OK and then open the Record window. You'll see a bar there like this:
Click on it and it will open a dialog window to set recording mode:
Under "Channels," you can set either "Multi Mono," which will give you individual mono files for each input; or "Multi Stereo/Mono" which pairs inputs logically (1 & 2, 3 & 4, etc). You can also set bit & sample rates here. Once done, click OK
Next, in the Record window, there is an Input section with a button on the far right (it may or may not have a number).:
Make sure the "Audio card input (hardware)" is selected and click on the button. Once you click on the button, you will see Recording Channels window:
Here you can choose the which of the available inputs you wish to use. Once you've selected the channels you wish to record, click OK and you will be returned to the Record window, looking something like this. Note the extra meter pairs in the Record window:
If you're recording in high rates, make sure to check the 'Split Mode' box and click on settings. Here you can enter the split argument. I normally set it at 1900 MB.
Keep in mind that I haven't gotten Wavelab 5 & the FW1814 to cooperate on the ASIO driver; but I've only been using a demo version and I'm not able to apply the ASIO patch Steinberg has. I think I'll get on to M Audio's tech support to see if I can find anything out from that end.