How do you select an option from a multiple option interatcive installer?

I'm sure it's obvious, but I have been pressing every key and can't select the "SDL12 and OpenAL source code" option in this installer. Pressing Enter/Return does either the OK or Cancel action. I have no idea what to call these things so it's been really hard to find the answer I need. It was a challenge to even come up with tags for this place. What do I do to / how do I select the "SDL12 and OpenAL source code" option? (and optionally, what in the world was I *supposed to search* to find the answer?)