Please note that this isn't a guide on how to earn the achievement but how to possibly fix it if you have already unlocked for it to not sync up to Xbox.com and correctly stick on your gamertag.
This should apply to following well known glitched achievements, I imagine it'd work for others as well if you've been unfortunate:
Mix It Up!
The Mix It Up! achievement in DiRT 2 (GFWL) worth 64 pointsPlay in every discipline over Games for Windows - LIVE (Pro Tour or Jam Session).With Great Honour
The With Great Honor achievement in DiRT 2 (GFWL) worth 59 pointsAchieve a 'Cautious' impact rating in Multiplayer races (Pro Tour or Jam Session).Five Of Your Best
The fix consists of rebuilding your Games for Windows Live profile and is simple to do.
What you'll need to do is go to the relevant folder in Windows for where this data is stored.
For Windows 7 / Vista it'll look like this:
C:\Users\Your Name\AppData\Local\Microsoft
For XP it'll look like this:
C:\Documents and Settings\Your Name\Application Data\Local\Microsoft
You should see two folders in here named "GFWLive" and "XLive". All you need to do is rename them, do not delete them.
A safe way of renaming them, as well as reminding yourself where they are if you need to restore them as backups is to rename them as such:
"OLD_GFWLive"
"OLD_XLive"
Simply put "OLD_" at the start of the name.
When you boot up the game, you'll need to manually sign in with your gamertag, what'll happen at this point is a complete download of your tag and its associated data as if you're redeeming it from scratch.
This will also create new "GFWLive" and "XLive" folders.
Once it has downloaded, you can try simply winning a race in single player. Once that's finished, the achievement should pop but this time should sync up to Xbox.com and therefore your gamertag!
Confirm this by waiting a couple of minutes then checking online.
Once confirmed you can do pretty much whatever you want with the backup folders, they're safe to delete at this point as you'll have fresh and more likely fully functional copies of the profile in the newly created folders.