Downgrading to an older version of Minecraft

1. Log in to the vanilla Minecraft launcher.
2. Click the "New Profile" on the lower left.
3. Name the profile "Minecraft 1.7.5 to play on nerd.nu" (or whatever else you'd like to identify it as being 1.7.5)
4. In the "Use version" dropdown, select "release 1.7.5"
5. Click the "Save Profile" button on the lower right.
6. Click the "Play" button with the new profile selected.