How to identify a greybox model on frd data?

I have a frequency response measurement of a MIMO system (8 inputs, 7 outputs) stored as a frd object. I know the internal structure of the underlying state space model and would like to tune my initial parameters by means of a greybox estimation.

The documentation for greyest() suggests that a frd or idfrd object can be used, but running the function returns the following error message: "Undefined function 'greyest' for input arguments of type 'frd'.". Does anyone have an idea where the error originates and how I should proceed?

I use R2013a on a 64bit windows 7 operating system.