i don't think they distribute binaries for mac os x, so more than likely you downloaded the source. if you have the
developer tools installed you can build bx yourself.
you need
dlcompatuntar the archive and configure with --without-gtk --without-ssl then run make and make install. usually things install into /usr/local unless you configure with a different prefix (--prefix=/somewhere/else).
it appears as thought
macosx.forked has c18 but not the newer c19. again if you don't mind building from source you can use
darwinports or
fink to do all the work for you...