Can't Export Song from Cubase 6 AI

Help me , i can’t export the song i arranged from Cubase 6 AI
when i tried to export it always show a pop up that “MP3 Encoder C” was expired
I Bought the Cubase 6 AI and activated it but the Mp3 Encode wasn’t activated and when it reach to the Max i can’t export again.
How to activated / reactivated the Mp3 Encoder again ?