SDLRoads is a cross platform remake of the Bluemoon classic Skyroads. OS X does not provide an easy way to provide command line arguments to a .app bundle.
For this reason, it is slightly more difficult to change tracks in this version of the game. However, it is still possible.
Simply edit the .sdlroadsrc file in your home directory (You can type "open ~/.sdlroadsrc" in a Terminal.app window to do this) and change the settings you want to change. The available tracks in the 0.09 binary are: bigtrack, track1 to track8, easy_track and testtrack.