type the code below into terminal. Until FTB likes mac you have to do this every time. Make sure the FTB_launcher.jar is in a folder named "FTB" on the desktop. (without quotes)
cd ~/Desktop/ftb/
export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home
java -jar FTB_Launcher.jar
Okay, the real reason this doesn't work is because FTB launcher does not work with Java 7, so you have to do this.