This article presents a hardware-in-the-loop (HIL) methodology developed at Politecnico di Milano (PoliMi) to perform wind tunnel tests on floating offshore wind turbines (FOWTs). The 6-DOFs HIL setup is presented, focusing on the main differences with respect to a previous 2-DOFs system. Aerodynamic, rotor and control related loads, physically reproduced by the wind turbine scale model, must be measured in real-time and integrated with the platform numerical model. These forces contribute to couple wind turbine and floating platform dynamics and their correct reproduction is of fundamental importance for the correct simulation of the floating system behavior. The procedure developed to extract rotor loads from the available measurements is presented, discussing its limitations and the possible uncertainties introduced in the results. Results from verification tests in no-wind conditions are presented and analyzed to identify the main uncertainty sources and quantify their effect on the reproduction of the floating wind turbine response to combined wind and waves.