"Secure iChat" is unavailable in OS X Lion
Encrypted iChats (also known as "Secure iChat") are no longer available in OS X Lion. The settings have been removed from iChat > Preferences > Accounts > Security.
You should use the built-in FaceTime app in OS X Lion and iOS 4 or later for secure video chats.
If you need to transfer a file securely using iChat on a computer with OS X Lion, you can encrypt the file and then transfer it with iChat.
Please note that iMessages sent via Messages in Mountain Lion are encrypted.