To get this to work, copy the folder "torque2d" into the PhysicsEditor "exporters" sub-folder. The location is different for each operating system. Once you installed the exporter, restart PhysicsEditor and you should see a "Torque 2D (XML)" in the "Data Format" drop-down.
You can also keep the folder in a separate location and in the preferences menu point to the location of the "torque2d" folder on your hard drive. See http://www.codeandweb.com/physicseditor/documentation for more information.
Note: All exported collision points (for Polygons) and radii and offset locations (for Circles) are in pixel units. Torque 2D uses meters as a world unit, so you will have to manually convert pixels into meters.