The <rankings> token displays the player rankings in a way similar to what you see on the Player Rankings page. This allows you to drop the rankings directly into your "clock" page, so you don't have to switch to the Player Rankings page (and away from the clock) to see the rankings.
This token displays the rankings in a table format, just like on the Player Rankings page. Scrolling it horizontally in a marquee would be strange at best. I think you're looking more for something that would list the rankings horizontally, with some additional text to make sense of it.
8 Corey 8:35 4 Ed, 9 Brad 8:22 3 Jack, 10 Adam 7:45 3 Jack
That, to me, doesn't make a whole lot of sense. But nevertheless, that's what it would look like if you took each row of the Player Rankings and showed them side-by-side. It's the titles at the top of each column that make it sensible.
8th place: Corey busted out at 8:35 round 4 by Ed, 9th place: Brad busted out at 8:22 round 3 by Jack, 10th place: Adam busted out at 7:45 round 3 by Jack
That makes much more sense. I'll add a new token to accomplish this.
To your specific question, the "tokens" in the Tournament Director are little bits of XML. To understand why your modification didn't work, you'd be best served by reading a little bit about XML. w3schools has some great tutorials:
http://www.w3schools.com/xml/default.asp