Steps for setting up RetroPi on the Raspberry Pi 2.

retropi.txt

1. Plug-in wifi, usb keyboard, power cord. Power up Pi.
2. Login with: pi/raspberry
3. Follow steps to install RetroPi at http://www.emulationstation.org/gettingstarted.html#install_rpi_retropie
sudo apt-get update
sudo apt-get install git dialog
cd
git clone --depth=0 https://github.com/petrockblog/RetroPie-Setup
cd RetroPie-Setup
chmod +x retropie_setup.sh
sudo ./retropie_setup.sh

4. Select the first option in the installer, "Install Binaries".
5. When RetroPi finally runs, setup the controls.
6. To download a folder of roms from dropbox:
wget -O ./roms.zip https://www.dropbox.com/sh/YOUR_URL_HERE?dl=1
unzip roms.zip
unzip '*.zip'

7. To enable HDMI audio sound (https://www.raspberrypi.org/forums/viewtopic.php?t=50086 and http://elinux.org/R-Pi_Troubleshooting#Sound):
cd /boot
sudo nano config.txt
# Uncomment:
#hdmi_drive=2
Then hit CTRL-O to save the file. CTRL-X to exit.

Next:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install alsa-utils
sudo modprobe snd_bcm2835
amixer cset numid=3 2

Type "alsamixer" to set sound volume to max level.

8. To run RetroPi, at prompt, type:
emulationstation

9. To modify controls:
cd /opt/retropie/configs/all
sudo nano retroarch.cfg

10. To exit a game hit Shift+Enter.