It's come to my attention that with the newer version of the Minecraft launcher, the instructions for adjusting your profile options are no longer relevant. I've updated the Development Environment instructions to reflect the changes.
Open up the Minecraft Launcher
Click on 'Launch Options' - the right-most menu item after News, Skins and Settings
Click on the Plus 'Add new' icon to create a new profile
Rename your profile so that it reflects the server version you'll be using, such as "Minecraft 1.6.4 or something similar
Under 'Version', select 'release 1.6.4'
Save your profile.
Click on any other menu option, such as News, Skins or Settings
If it isn't selected yet use the upwards-facing triangle to the right of the 'Play' button to select the new profile
Click 'Play' to get started!