Android 5.0 is a new age for rooting on Android. Google's latest security enhancements require more hackery to circumvent and the Nexus 9's 64-bit software complicates matters even further. Still, after just a few hours, Chainfire has updated SuperSU to work on the Nexus 9, but there are a few extra steps.
The nature of root on Android 5.0 might make the CF-Auto-Root tool trickier to get working, so for the time being this is a manual process. Here's the gist of it.
- enable oem unlocking in Android's developer options
- fastboot oem unlock
- fastboot boot inject.img
- fastboot flash boot patched.img
The necessary files are available on Chainfire's site, but be aware v2.18 of SuperSU is a custom build for the Nexus 9. It should not be flashed on any other device. As usual, use caution when flashing stuff to your tablet. At least there's a factory image now that will fix most of your mistakes.