Marco.,
I understand the SEMW Auto Levels plug-in requires Movie Studio Platinum 12 or 13, where they do show up for me, but not in VMSP 11.
The OP said he has Movie Studio 11.
Understand that this is based on the loudest point. If you have a very loud point in an otherwise low level audio track it will raise the entire track up so that the loud point is at the set point, but the rest may not be as loud as you wish.
You may need to find those "loud points" and manually reduce them so that the normalize function brings up the rest to your liking.