Download the MaximumMinecraft disk image file.
To install MaximumMinecraft, double-click the downloaded DMG file & drag and drop the application onto your hard drive. I would suggest the Applications folder, but it should work anywhere.
On first run, MaximumMinecraft will set its preferences with interactive dialog boxes. To change those preferences at any time, delete the MM_Preferences.txt file in your /minecraft directory.
Once preferences setup is complete, MaximumMinecraft will give you the option of quitting or continuing on to launch Minecraft with the preferences you've selected.