In order to unlock this achievement, you must have 266 XP points. XP points are earned in online races, either player matched or rank matched. Following is a chart saying how many points you get depending on how you place (Last place always gets 0 points):
2 Players: 28 - 0
3 Players: 30 - 14 - 0
4 Players: 32 - 20 - 12 - 0
5 Players: 34 - 24 - 16 - 10 - 0
6 Players: 36 - 26 - 20 - 16 - 10 - 0
7 Players: 38 - 30 - 24 - 20 - 14 - 8 - 0
8 Players: 40 - 32 - 28 - 24 - 18 - 12 - 6 - 0
You can check your XP by going to
http://games.ea.com/nfs/carbon/us/profile.jsp?platform=xbl2&...(Replace the XXXXX with your Gamertag, spaces omitted).
On occasion, you will stop gaining XPs. This happens if you are playing the same person or group of people for a prolonged period of time. You can either try disconnecting mid-race to restart the XP process, or try hooking up with a different group. Use the website listed above to keep track of your XP and whether or not it is building.