On the taskbar , the second icon from the left (looks like a manila folder), is the PCMan file manager. Click on that to open it and in the top left pane you might see a list of different sized volumes, one of which would be your drive. If it does not appear there, then it is possible Knoppix is unable to mount or open it, You can try mounting it manually as described above.

I thought I would add this as the use of the file manager is not particularly intuitive to a windows user, and the use of the command line can be daunting at best.