Go in when its quiet,
get a burger (or at least something to make it look like more than a token order, drink and a McFlurry or something), ask greasemonkey behind the counter about it. They'll happily give the info to a paying customer, and I doubt they change it regularly.
Though a quick internet scanning seems to suggest that it cuts you off after an hour or so, still worth a shot though
Edit: its possibly some form of voucher you get after buying which gives you a temporary login, problem is the kind of people that eat at McD's aren't really the kind given to decent expalinations
( Tedium Alduce me to interlow myself., Wed 5 Aug 2009, 23:44, 1 reply, 16 years ago)

It's ok, I asked on another forum I use and they found me webshots from somewhere :)
Looks like it was just being a bit temperamental when I tried it
( fuck shit up the best you never had, Wed 5 Aug 2009, 23:56, Reply)