Would that command be from within the psql SQL Shell that came as part of the install? (I'm living in Windows land).
if you're already in psql, logged onto your database, it would be \i filename.sql
psql -f filename.sql dbname... would be at the system shell prompt, but that assumes the postgresql binary directory is in your path, which it may not be on default Windows installs.
-- john r pierce 37N 122W somewhere on the middle of the left coast