I've see this Apple post already:
To manually set up your computer to use IPv6
You should only do this if your network administrator or Internet service provider has specifically told you to manually configure IPv6.
- Choose Apple menu > System Preferences, and then click Network.
- If the Network Preference is locked, click on the lock icon and enter your Admin password to make further changes.
- Choose the network service you want to use with IPv6, such as Ethernet or AirPort.
- Click Advanced, and then click TCP/IP.
- Click on the Configure IPv6 pop-up menu (typically set to Automatically) and select Manually.
- Enter the IPv6 address, router address, and prefix length you received from your network administrator or Internet service provider. Your router address may be referred to as your gateway address by some ISPs.
(source: http://support.apple.com/kb/HT4667)