Just drag in the Finder. The folder containing FBtoC.app must also contain three special folders: build_goodies, Headers, and User Libraries.
build_goodies contains pretranslated and precompiled parts of the runtime. Changing any of these files is likely to break FBtoC.
Headers contains a subset of the standard FB headers files. Most of them needed modification (sleazy special-constant hacks) to make them work with FBtoC. Some files are placeholders.
User Libraries has the same function as in FB, except that the little-known special file UserFloatPrefs is ignored by FBtoC.