I can find VMRC SDK for vSphere 5.5 on VMRC SDK for vSphere 5.5 - VMware Developer Center but not for 6.0.
VMware Remote Console SDK Documentation also doesn't mention where to download SDK.
Anyway, this blog by William Lam describes the correct usage of VMRC API.
Simply open url "vmrc://clone:CloneSessionTicket@vCenter_IP:443/?moid=VM_MOID" in web browser will launch VMRC application and show the specified VM console, where TICKET is obtained by calling the AcquireCloneTicket() method using the SessionManager in vCenter Server.
As of vSphere 6.0 the VMRC SDK has been discontinued. From the VMware Remote Console (VMRC) SDK Documentation page:
VMRC 5.x plug-ins were replaced by native Remote Console or Web browser console, and the SDK by VMware HTML Console SDK.