did you try what Thymox say then??
If it didn't work, well this may be because your initial configuration of your system did not include USB support for your system, in which case you would either have to re-install your entire system or recompile your kernel.
What version of mandrake are you using?
If I'm not mistaking, on the latest kernels (2.4.17+) USB comes supported as default.
Anyway, tell us what's going on,