Noteworthy composer is a Windows program, but it will run on Linux using Wine.

Wine is also available for Macs, but you have to install the source code. There are clear instructions on the Wine website, but it is not for everyone.

http://www.winehq.org/MacOSX/Building

I am a little wary of compiling and building from source code, but I had to do it recently to install a Linux sound driver on my new laptop and it turned out to be quite straightforward. Just a matter of following the instructions given.