Step 1, make a folder called ftb in your desktop and put your ftb launcher.jar into there.
Then in the options change the install folder to the folder on your desktop. If you do this you will have to reinstall ftb, but it is worth it.
Then enter this into terminal, the whole command, and nothing less:
Code:
#!/bin/bash
cd "$(dirname "$0")"
java -Xmx1024M -Xms512M -jar *DRAG YOUR FTBLAUNCHER.JAR INTO HERE, BE SURE TO DRAG THE ONE IN YOUR DESKTOP FOLDER CALLED "ftb"*
If you want to automate this, you can make a start.command file with this in it and then give it permission.