H.264 grayed out in export menu

I am trying to export from FCP 5.1.4 to H.264. I select "Export using Quick Time conversion," MPEG-4. However, when I click options and bring up the "MPEG-4 Export settings" H.264 (and pass and pass through) are grayed out, so I cannot select them. The same is true in Quick Time. I have Quick Time Pro 7.4.5.

Why is H.264 grayed out, and how can I make it so that I can select it?

I feel like I'm missing something obvious.

Thank you.