As David points out the only way to Export a Terramodel PRO file is to use the Exports to SCS900 / Siteworks in the Field Data tab of the software. The Design File exporters that contain an Alignment or a Road Surface Model are output as PRO Files.
The PRO Files will end up in a Design Folder for the SCS900 / Siteworks device that you assigned the Design to and will be in the Trimble Synchronizer Data folder (normally on your C:\ drive) under the Device Name and then Trimble SCS900 Data and then the Project and then Designs and then Design Name.
Alan