I'm not familiar with either Mandrake or USB cable modems, but I can make a couple of suggestions. The site with all the Linux USB info is www.linux-usb.org
which unfortunately does not list the RCA 235, although it has some other cable modems. Not a real good sign. What do you mean when you say Mandrake 'sees it'?
Do you know how to open a terminal window and run programs from the command line? If not maybe start with a good beginner guide e.g.
Then you can do some useful things. e.g. command 'lsmod' lists all the driver modules linux has loaded. You could see if there is any sign of your modem there.
Assuming you can somehow install a driver for the modem (this looks like the hard part) you may be able to configure it from the command line using the 'ifconfig' command. In fact typing 'ifconfig' on its own will list your network interfaces. I suppose any USB network connection would show up there.