It may depend on the version of PPT. Some versions include import filters for
Harvard Graphics and Freelance PRE (but not PRS files or PRE files saved from
later versions of FL).
If you can't use File, Open and choose PRE in the Files of Type: dropdown list,
then try re-running Setup from your Office CD. Choose a custom installation
(or Add/Remove features, depending on what it offers) then explore for the
FL/Harvard Graphics options.