There are currently two revisions of the .fp file format (you can see the gory details
here, if you're interested. It's described in section 6.1).
The later format (referred to as "5.1" in the document) is supported by CVI version 5.5 and later. If whoever created this .fp at Rohde & Schwarz used the later format, then you do need to have CVI version 5.5 or later in order to be able to use it.
To confirm that that is indeed the problem, you can download an eval version of CVI 8.1
here to see if you can use it in 8.1.
Luis