The Cactus user guide does not explain how to use boolean parameters from Fortran.
There is an explanation present since at least git hash 3c44bc84 "Fixed numerous spelling errors, some grammatical boo-boos, formatting flaws (inlcuding gratuitous hyphenation and un-dotted 'e.g.' and 'i.e')" of cactus which is from Mon Jan 5 16:28:24 2004. This is in doc/UsersGuide/ThornWriters.tex and currently reads:
All parameters defined in a thorn’s param.ccl and all global parameters, appear as local variables of the corresponding CCTK data type in Fortran source code, i.e. Booleans and Integers appear as CCTK_INT types (with nonzero/zero values for boolean yes/no), Reals as CCTK_REAL, and Keywords and String parameters as CCTK_STRING (see also note below).