FBtoC works on OS X 10.3.9 and later.
Before using FBtoC you should have installed Apple's free Developer Tools (a.k.a. Xcode Tools), from your OS X install CDs or DVD.

To create universal binaries, you must have OS X 10.4 or higher, gcc 4.0 or higher, and install the Cross-development SDKs. Without /Developer/SDKs/MacOSX10.4u.sdk, only the native architecture of your Mac can be built. The Finder view (below) of the Developer folder shows the SDKs after installation. Cross-development is an optional part of the Xcode Tools install (from the OS X DVD or CDs).

See also Apple's documentation: http://developer.apple.com/documentation/DeveloperTools/Conceptual/cross_development/Introduction/chapter_1_section_1.html