Okay, a couple things. I'm not sure why you have the StockSurface set at 8mm but it should be zero. Your part is already positioned so that the top of the part is at Z0 so your good there. This is the normal practice. Setting this value to anything more than zero will result in a lot of air cutting.
The next thing you need to do is re-size the part. If your click on the model from your file and then click Tools:Get Object Extremas, you will see that your model is much smaller then when you designed it. From your post, you said your part is 6MM thick.
Click the model then right click, Transform:Resize. The resize dialog box will pop up. Make sure the 'Preserve aspect ratio' is checked to keep everything proportional. Simply type 6 in the Z then apply and everything else will automatically update. So if I assumed correctly your part should measure 20x20x6.
The last thing to do is to define your stock. This is pretty straight forward and needs little explanation. If your unsure about how, I can go into detail for you.
Now just setup a 3D MOP as normal and everything should work out.
The main issue was that the part was so small that it made the tool path look huge and out of place. Now I'm not sure what size cutter you had in the MOP but I attached an example with a 3mm end mill 50% step over.
If there's anything else I can do let me know.