If you have an existing USB or PS2 K500 keyboard currently programmed to work with Myriad 3 or Myriad 4 then you can re-program it to work with Myriad 5 and even update the graphics for under the key caps (as long as you are OK using scissors!)
First, download and install the ChangeMe software.
Then download the 2 attached files which are the Layout File and the Button Graphics PDF file.
Plug in your K500 before you run the ChangeMe software, then load up the Myriad 5 Layout and 'push' it to your Myriad keyboard.
You can also update your key caps with Myriad 5 labels using the layout PDF file.