This achievement is quite clear: Win all the real events, and unlock the next by completing the already unlocked ones.
HOWEVER: Many people are having a game crash/freeze issue whilst playing Real Events, for those, I write this guide here.
There is an easy fix for this. Using your GPU app, force limit the max framerate for the game to max 120.
For NVIDIA: nvidia-configuration > 3D-settings > Program-setting > Add (browse to your Steam folder ./steamapps/common/MXGP2 - The Official Motorcross Videogame\ And here select the correct ".exe" file of the game.
Now scroll down to: Max. framespeed and Max. framespeed backgroundapp.
Change both to 120 FPS
Apply and launch the game, Real Events should now work.