Freewheel mode can be activated by setting Axis Parameter 204 to 1. This setting only takes effect if StealthChop is active. No other chopper modes will allow you to enter freewheel mode.
As an alternative to freewheel mode, you can also set Chopper Off TIme (axis parameter 167) to 0 in order to completely disable the driver and thus, the motor will freewheel.