There are some situations where it will be grayed out. You need to either click on a message in the message list (without subsequently clicking in the body of the message without selecting some text), or select some text in the body of the message display. With the default Mac OS X color scheme, if you have a blue highlight on the message in the message list, it should work, and if you have a gray highlight, it won't, unless you have selected some text in the message.
Good:
Good:
Bad:
