Have you installed the app to /Applications/ ? That is, made it an app available
to all users? If so, then you can try
1. Create a new user account
2. Log in to that new user account
3. Launch ReplicatorG
If it does the mysterious hanging, then it's an issue between RepG and the
environment presented by your operating system in its current state. (I'm
a mac user; works fine for me.) If it does not hang, then it's something to
do with past usage from your account (e.g., trying to open and display
a model which is giving it grief). Also, when you launch RepG it tries to
make a network connection to
firmware.makerbot.com in order to download
firmware. It shouldn't hang when it does that, but I thought I'd mention it.
It's also possible that something is appearing in the console log about a
problem. That's the console app in the utilities folder of the applications
folder. You can select "all messages" on the left and then fire up RepG and
see if anything tell tale is logged.
Dan
P.S. Or you can make a new account and just install RepG for that account and
try it.