Exploration of virtual worlds with unconstrained locomotion possibilities for the user is the main objective of the European research project CyberWalk. This should be achieved through the use of an actuated platform (the CyberCarpet) that compensates for the walker's locomotion in such a way to keep her/him close to the platform center. This paper presents the control problem for the platform motion, including objectives and constraints, overall control architecture, and kinematic modeling. Since the platform has only two actuating devices (linear and angular), the control problem is similar to that of output regulation for nonholonomic wheeled mobile robots in the presence of an unpredictable disturbance due to walker's locomotion. Based on the kinematic model, a velocity control design achieving input-output decoupling and linearization is proposed and its performance is verified by simulations