Workspace frames are available when using at least Windchill 9.1 and Wildfire 4.0 or later. The preference already mentioned is found in the Preference Manager under Windchill Workgroup Manager > Client. If you set the preference, Pro/E will need to be restarted for the change to take effect.
Information about the preference can be found here: https://www.ptc.com/appserver/cs/view/solution.jsp?n=CS42274
