[MENTION=754954]knav2013[/MENTION] I think you need to unsubscribe from the thread(s) you want to change, then re-subscribe to them with the Instant option selected. As far as I'm aware, there is no way to just change "None" to "Instant" on a particular thread.

Wait! I'm wrong... check the box next to the thread(s) you wish to change, then near the bottom of the screen you'll see "Selected Threads" and a dropdown box, change the dropdown box to "Instant Email Notification"

(I believe there was a time where you had to unsubscribe then resubscribe, but apparently they've fixed it since then! ^)