If you're developing only music, you don't need to worry too much about the actual spring files, but right now the best way to get openmachines is through sourceforge.net/projects/openmachines . You need to get the link to put into mercurial (Basically an organizer for software development) and just use the link to "clone" the repository. the files in the repository should all be put under a folder that ends in ".sdd" so that you can put the folder under "mods" in the spring directory, and if it works, then you should be able to choose openmachines as the mod you want to play, when you boot spring's lobby client (you can choose between three lobby clients; I use springlobby).
Yes, this is annoying and complicated. We can make it much simpler, but I was advised against putting effort into making it easier to run such an underdeveloped game >.>