Jeremy, You didn't tell me you were using SDL as well, otherwise I would have provided more info.

MacOS has the concept of "Frameworks", which are essentially application bundles for shared libraries. So here is what you need to do ( and has worked for me )...

1. grab the SDL dmg files from the libsdl.org site and install them. There will install the SDL Framework.

2. In your Lazarus project options (-k if you are using the command line) on Linking, you need to pass the following...
Code:
-framework carbon -framework cocoa -framework sdl
obviously add similar things for sdl_mixer, sdl_image etc etc.

3. Again in the Lazarus project options, on the Other tab in the "Custom options" section you need...
Code:
-XMSDL_main
This is required because the SDL Framework requires a C style "main"
and means you need a file called SDLMain.o which contains a main function called SDL_main. There may be a better way to do this, so if anyone find out how to do it, please let me know.

4. in your project file you will need
[pascal]
{$IFDEF DARWIN}
{$LINKLIB gcc}
{$LINKLIB SDLMain}
{$ENDIF}
[/pascal]

So the only thing left now is a missing SDLMain.o file.