I set up there once a couple of years ago; went Friday night and stayed over. You just go find an open spot. I probably won't set up there again but might go to shop sometime. The spots are measured and they check to see that you don't get a foot out of your allotted space; that was okay, though. What I found (which may not be true every time) was that people are really looking for bargains, so it isn't a place to take your quality birds. If I priced a full-grown chicken at $5, people tended to shake their heads and move on. I spent more on gas and the $20 set-up fee than I took in.
I prefer the Knox and Milton KY (just across the river from Madison) swaps. They are smaller, but I have always done well at them, usually selling several before I even get them all out of the truck. Folks have even helped me unload just so they could see them first. Knox is pretty far for me, but I really enjoy it--hope to go in June. My friend who went with me last year took in about $500, and we only stayed about 5 hours.
The Milton swap will be May 11-12 and Labor Day weekend; Knox is the 2nd Sunday of the month April, May, June, September, and October.