Hi, just registered to thank everyone who had some input on this.
As mentioned above, I was able to get the Poseidon Z working on OSX El Capitan by first disabling System Integrity Protection, which locks certain directories out of writing even for root for unsigned kexts.
1. Reboot Mac and load recovery mode (cmd+r on boot)
2. Open Terminal
3. # csrutil disable ; reboot
After disabling SIP, I was able to follow the original instructions to install the kext. It's working pretty good now!