OpenVPN JSON error after upgrading to El Capitan
Short answer, you can't.

However, at work we had this issue.  Some machines that previously had OpenVPN installed prior to upgrading to El Capitan had no problem.  

Others required that we run the uninstall script for OpenVPN.  Then disable SIP, install OpenVPN, then re-enable SIP when you are done.  Annoying, and a bit time consuming, but the end result is that you have OpenVPN installed and SIP is enabled.

This has worked on multiple Mac's at our office.

The other alternative was to use Viscosity which does not require SIP to be disabled, but has a small licensing fee.  I have heard that Tunnelblick also works, but have not personally tried it.