Please see http://www.linux-usb.org/USB-guide/x194.html
as you have two things going on. One, you have to check that your USB hardware is being discovered, and second, you have to configure your "X" windows to know how to treat the USB device.
I'm not using any USB KB or Mouse, but the above document looked pretty straight forward, but be prepared for lockups while configuring X-windows on the new mouse/keyboard combination.
You might look into using single user mode on your system as you mess with it, see your /etc/inittab file, specifically,
# Default runlevel. The runlevels used by Mandrake Linux are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
so changing to
will allow you to manually start X-windows while you mess with it, then you can change it back to init 5, when things are running ok.
(see startx to manually start X-windows).