You didn't mention what kind of usb device it is.
There is a libfdti but it seems to be intended for embedded devices. On the client side I don't think it would matter.
Look up documentation for Policy Kit & HAL. KDE receives messages from HAL via dbus. Udev should already create a device for you. "lshal -m" will monitor the messages sent when you insert a usb device. You can also use "udevmonitor." Is there already an .fdi file that covers it? Do you know what kernel module is used? You can insert the device while monitoring the kernel messages with "sudo tail -f /var/log/messages". You can also use "udevmonitor -env" to monitor udev and kernel environment values.
Three things to look at are:
- Which kernel module needs to be loaded.
- Do you need to install an .fdi file for hal to function properly with this device.
- Do you need to add a udev rule to create the device node.
If your system uses Policy Kit, there may need to be a policy added as well, but it may be included with the .fdi file.