Better WAY. As of 20th August 2010 ALL Leaderboards are blank after being down for a while. Basically make a blueprint (by even just tuning a car). Race ANY raceday as solo and just do 1 race, that will get you in top 100 (I came last in the default car, it was recked car and I still got Number 1).
Exit to menu after the race and then re-race the same race day with your blueprint (does not even need to be the same race). It popped for me straight away.
UPDATE-Check the weekly leaderboard for each race-day. As some have fewer then others (1 yesterday had 3 people on it, so getting in the top 100 would be easy).
If you struggle with above a few people have used the method below: ALL CREDIT FOR THIS IS TO "Hey Who Sharted", i thanky you:
"I just used my 240SX, raced in Battle Machine-Willow Springs, it has 8 different events - 3 grip, 1 sector shootout, 1 time attack, 1 1/4 drag, and 2 drift events. I completed the first 3 grip events and the sector shootout and after I completed the sector shootout the achievement popped for me"