Most of the ROM hacking tools we use are for Windows computers. There are a number of tools that are being developed to be OS-independent, but for the most part you're out of luck. Some tools, mostly those written in Python, do exist: Red Alien (a script editor) and Blue Monkey (a map editor) by cosarara and G3HS by karatekid552 were written in Python and should all be usable by a Mac. If you install Mono you may be able to get some newer tools running that were programmed for .NET, but there's no guarantee. Fortunately, there are hex editors written specifically for Mac (0xED comes to mind), and if you know what you're doing a hex editor and an emulator are all you'll really need for basic ROM hacks.