Hi David,
I will test this on my end also but some times an OS version update will introduce "ghost" HID's, especially keyboards or trackpads, that can be opened but do not generate data. This has happened a few times before. The way to check is to change the kbNum and see if using a different number (1, 2, 3, etc.) produce data. You only need to try this until you see "couldn't open keyboard #" for whichever number after 0, which means you've exhausted the list of things ChucK thinks is a keyboard.
Otherwise this could be a new issue specific to El Capitan :(
spencer