Hello Christian,
All of the DesignLab libraries are based on VHDL or Verilog and just uses the schematic editor as a way to let people be productive on an FPGA without learning VHDL/Verilog. But if you already know it then you are going to feel at home, you can right click on any schematic element and be right in the VHDL or Verilog to make your changes. You can also choose to not use the DesignLab environment and just use Xilinx ISE directly with the provided ucf files and the Papilio Loader to load bit files. The DUO is like any other Papilio FPGA board in that respect.
To find the documentation please visit (Well I’m a new user here so I can’t post the links in a friendly format):
duo . gadgetfactory . net
learn . gadgetfactory . net
compshield . gadgetfactory . net
logicshield . gadgetfactory . net
Also, please feel free to drop by the Gadget Factory forum and participate.
Thanks!
Jack.