The manual and the FAQs mention the 'SSH Sentinel' software that installs and runs on the remote PC wanting to connect to the 550, however I can't seem to find that software on the included CD to load.

The Sentinel client software does not come bundled with the RF550VPN, it is an optional purchase. You can contact your local distributor and purchase the Sentinel client software.

Here is a URL contains part number for the VPN client software:
http://www.multitech.com/en_US/SUPPORT/Families/RouteFinder/firmware