I'm running OS X 10.10.3 and am able to access my own Hikvision cameras in Safari with the plugin. I can also access my brother's year old NVR DS-7604NI-SE/P that has the V2.3.7 firmware.
However, I can not access my other brother's brand new DS-7608 - keep getting the plugin is not installed message.
It definitely appears that Hik is not keeping the plugin up to date with their newest firmware. Dang...