That Forest version is too old. Please, update to Forest Pro 3.2.4. You will need VRay 1.5 SP2 or SP3 as well.
Actually, is not needed to instance vray proxies. Forest does it automatically. It's much faster and you can render millions of objects.
By the way: for questions about Forest Pro, please post in Restricted Boards->Forest Pro General. You will need the username and password provided with your Forest license to write there. Thanks.