See https://developercenter.vmware.com/web/sdk/60/web-client
Before posting questions in this forum please read docs/Release-Notes.html in its entirety :-)
If you are having problems with an existing 5.5 or 5.1 plugin on vSphere 6.0 make sure you followed the compatibility warnings. As always, be very precise with your problem description, and if possible try to reproduce bugs with one of the samples.
Thanks and enjoy 6.0!