Nicolas,
Bring your router down such that the tool just touchs the top of the piece. That is Z=0.
In the Layers Tab menu where you set the cutting parameters, set the rapid height to say 1.0 ( LC defaults to this value, the Cut Start at 0.0 and the Cut Depth at MINUS -0.1.
The machine rapids at 1.0 above the work piece, it stops at some location, it then goes down to 0.0 where it thinks the cutting is starting, it then continues .1 below the surface into the material.
Make sure that you are putting the -.1 value in the Cut Depth box and that you hit the enter key on the keyboard.
You should see -.1 in the Cut depth box. Click Set All Layers button as this sets those cutting paramters for all the enabled layers.
Look at the screen display and you should see that the tool is cutting below Z=0. After posting to Mach,and your now in Mach, make sure you reference the axis. Referencing the Z will set Z=0 in Mach. Look at the display in Mach and just go down through the code line by line and you will see all the moves.
Just echoing what has already been posted,
RICH