The javadocs at http://www.frege-lang.org/doc/index.html seem to be missing quite a bit (namely, no frege.prelude package), and does not seem very up to date. Might there be a good documentation for Frege hidden in plain sight somewhere?
What you have seen is the API doc of the runtime classes, which is really minimal since the typical Frege user will not work with those.
Thomas correctly pointed to the library documentation.
In addition, there is "Hoogle" support for Frege: http://hoogle.haskell.org:8081/ where you can search the API.
All is linked from the Frege home page.
Hope that helps.