The link from the post above didn't work for me, but I do know there are some Altera-provided resources on the nios ii literatures page (http://www.altera.com/literature/lit-nio2.jsp). Specifically, see the ones below:
The Custom Instruction User Guide is here:
http://www.altera.com/literature/ug/ug_nio...instruction.pdf (
http://www.altera.com/literature/ug/ug_nios2_custom_instruction.pdf)
The accompanying design files for the Nios II Cyclone board (1C20) are here:
http://www.altera.com/literature/ug/niosii_cyclone_1c20.zip (
http://www.altera.com/literature/ug/niosii_cyclone_1c20.zip)
Additionally, you may find something useful in the nios ii online demos (
http://www.altera.com/education/demonstrations/online/embedded-processor/onl-nios2.html).
Good luck.