There are several reports on Diablo 3 initializing being stuck at 0 or the launcher refusing to start. This is affecting all systems including Mac, Windows and other platforms. Some solutions or fixes has been posted on forums.
Here is a solution post on Battle.net:
PREREQUISITES FOR THIS TO WORK:
- your system volume is case-sensitive (see below how to check)
- you have another hard-drive at hand other than the case-sensitive one
0) Start the Diablo III installer and wait for the “Cannot initialize streaming” error message. This will create some files and folders that will later have to be moved.
1) open Disk Utility and look at the volume your system runs on (usually called Macintosh HD) – if you find “Case-sensitive” in the description in the lower left part of the window, then this tutorial is for you and will fix your issue
2) now that you are in Disk Utility press “Repair Disk Permissions” just for good measure, it will take a couple of minutes, you dont have to wait for it stop before continuing on
3) open the Terminal app and type in “cd /Users/Shared” and then “ls” – I did have 3 Blizzard related items there: Battle.net, Blizzard Entertainment and Blizzard, this might be different for you (if you don’t play SC2 or WoW), the most important one is Battle.net, but I will treat them as equal
4) now you will need to have another HDD install Diablo 3 on, this one has to be case-insensitive, in my case I have a second internal HDD which is mapped to /Volumes/HDD
5) now lets prepare a directory on our case-insensitive drive, type the following into terminal
mkdir -p /Volumes/HDD/Users/Shared
mv Battle.net Blizzard Blizzard Entertainment/ /Volumes/HDD/Users/Shared/
NOTE: replace /Volumes/HDD with /Volumes/whatever-your-second-hdd-name-is, the first command may need admin rights, make sure you are an admin!
6) the final step is to create symlinks so that the installer is able to find these dirs, copy these into your terminal window
ln -s /Volumes/HDD/Users/Shared/Blizzard Blizzard
ln -s /Volumes/HDD/Users/Shared/Blizzard Entertainment/ Blizzard Entertainment
ln -s /Volumes/HDD/Users/Shared/Battle.net/ Battle.net
7) make sure that your Diablo III installation files are also located on the case-insensitive volume
8) start the Diablo III Setup and enjoy the game 
or, as I do now, enjoy the login screen of the game
If this does not work; try re-installing Diablo and make sure your on an online mode.
