Here is the full list of all 15 MXGP - there are The Official Motocross Videogame Compact achievements.
Get enough Experience Points to reach MX Level 5
Get enough Experience Points to reach MX Level 10
Get enough Experience Points to reach MX Level 20
Enter the Bike Set-up menu in the Pit
Select the Pro Physics setting in the Riding Assists section and complete a valid race
Enter Replay mode for the first time
Get pole position in an offline Qualifying session
Ride for a total of 300 km (187 miles) in any offline mode
Spend a total of 2 minutes airborne from jumps in any offline mode
Get pole position and win both races in the same offline Grand Prix
Win a race starting from the last position
Complete an Online Grand Prix
Get the Holeshot in any offline mode
Get 10 Holeshots in any offline mode
Complete one valid race on each track in the game in any offline mode