So I spent the past few days trying to get Ubuntu Studio installed on my 2006-era Mac Pro 1,1. I can't speak for other Macs specifically, but here are some details you're going to want to know if you engage in that undertaking:
- The Mac Pro 1,1 won't boot Linux from a USB stick.
- It also won't boot it from a dual-layer DVD. Double-check and make sure you're not using dual-layer.
- The LTS releases of Ubuntu (such as 14.04) have images that are specifically labeled "amd64+mac". Use those. Otherwise you might wind up stuck on an unresponsive "Select CD-ROM Boot Type" prompt.
- You may or may not need to install rEFInd to help you boot from a Linux disc. If your disc isn't showing up when you hold the Option key at boot, give rEFInd a shot.
- There's a useful guide at Ubuntu Community Help called Installing on a Mac Pro - Cylinder (Late 2013). As the title implies, it's not written for the older-model Mac Pros, but most of what it says is still applicable. (But it tells you not to use the Mac-specific ISO files. Don't listen to that part; you should use those on the 1,1 model.)