So how do i use those Pins? I noticed, that all Arduino Pins are connected to the Motor Shield, and that next to the connecting Pin's there is another row of unused "holes" ;)
Is each of these holes connected to the corresponding Pin? So can i just use those holes to solder in some male headers and use them to connect Sensors?
Yes. There are breakout holes next to the Arduino headers. You can solder you wires there directly, or solder in more headers if you like.
I notices 2 more rows of holes, 6 holes each. Is it correct that one Row is connected to 5V and the other Row to GND?
The triple row of breakouts next to the analog pins also supply +5v and GND for plug-in compatibility for many 3-wire sensors.