The problem is that the n900 only supports ad hoc wifi tethering or bluetooth tethering but the Touchpad doesn't support any of them out of the box. You can do one of two things:
1) If you want to stick with webos: root the Touchpad, install the apps necessary to open a terminal, and run a few commands to setup the webos wifi to have it attached to an open n900 wifi signal
2) If you are okay with it: install either CM7 or CM9. Either of those should allow for you to create a secured ad hoc wifi connection to the n900. I haven't tried it yet, but I'm guessing bluetooth tethering should work using CM9.