This tutorial is for those who like to do things in the terminal. If you're not one of those people, go check out iDog's iBurn 360, an all in one GUI application that does everything you need. You can get it here: http://idog.appleguru.org/ There's even a manual 
If you happen to be someone who likes the terminal, or you just want to learn more... read on.
First and foremost, thanks to cyberdork33, milocoon, 2gunsup, and Pandor over at xboxhacker.net for supplying the information to make this possible.
Requirements:A Mac running Mac OS X
A dual layer DVD burner
Software:growisofs:
growisofsInstructions:1) Download the growisofs installer and run it.
2) Open up the terminal application; you can find in in the Utilities folder in your Applications folder.
3) Type
ls /dev/rdisk? in the terminal window and hit return.
It should output something like "/dev/rdisk0".
4) Put a CD/DVD (NOT BLANK!) in your dual layer dvd drive. Wait for it to mount.
5) Once the disk mounts, type type
ls /dev/rdisk? in the terminal window again and hit return. You should see an additional entry this time. This additional entry is the address to your dvd burner.
6) Type "
file " without quotes (notice the trailing space). Drag your Xbox 360 disc image (.iso, .000 depending on how you made it) to the terminal and hit return. If it says your file is "data" then go to step 7. Otherwise if it says you have an ISO (Ex, ISO 9660 CD-ROM filesystem data 'XBOX360'), skip to step 8.
7) Do this step ONLY If you see type "data" in step 6. Type "
java -jar /usr/bin/imgbpatch.jar " without quotes in the terminal (again, notice the trailing space). Drag your Xbox 360 disc image to the terminal again and hit return. Once it says "Finished!" go to the next step.

Now put a blank dvd+r dl disc in the drive. Click
ignore when the finder asks you what you want to do with the disc (If it doesn't ask you, go to system preferences --> CDs & DVDs and set it to ask you).
9) Copy and paste the following:
growisofs -use-the-force-luke=dao -use-the-force-luke=break:1913760 -dvd-compat -speed=2 -Z /dev/rdiskX=Where X is the removable media device number we got in step 5.
10) Drag your Xbox 360 disc image to the terminal again.
11) Hit return and wait for it to finish (Should take ~ 47 minutes @ 2X... You'll see a bunch of text on the screen; you know it's done when you get kicked back to the command prompt, after seeing "/dev/rdiskX: closing disc")
Notes: If you have a faster drive (say an external one) that supports writing DVD+R DL @ speeds faster than 2.4x, feel free to change the speed to 8X or whatever your drive supports.