MDK Professional contains multiple example projects that show how to implement an USB Device. The following examples are available for most of the development boards:

The USB Device HID example provides access from the USB Host to the development board's LEDs and push buttons
The USB Device Mass Storage example provides mass storage device to the USB Host and uses SD/MMC memory card or internal RAM as storage media
The USB Device Virtual COM Port example provides virtual communication port to UART bridge to the USB Host
The Ethernet-over-USB (for Linux hosts) example shows how to connect an Ethernet device via USB (for Linux hosts)
The USB Device RNDIS to Ethernet Bridge example shows how to connect an Ethernet device via USB (for Windows hosts)
The USB Web Server (for Windows hosts) example contains a step-by-step instruction how to change a network application example so that the embedded device can be connected to a Windows host using USB.
The USB Composite Device example shows how to create a composite device using the HID and MSC classes
The USB Mouse example shows how to create a USB device that acts as a mouse input device
The USB Device Audio example provides an audio input/output functionality to the USB Host
The Custom USB Device (WinUSB_Echo) example provides an example for a custom USB class

To use these examples, use the Pack Installer, select the related Board and Copy the example.