How to export the milling layer properly with Eagle 8.0?
On EuroCircuits, you have to do the following:

1 - Change the milling length you desire (left menu on the PCB Viewer):





2 - Access the "Milling Editor" to select the desired outlines to be milled (right menu on the PCB Viewer).



Here you select the layer on which the outlines are, and then click on the outlines. Its quite straightforward but a bit of a lengthy task on PCBs with lots of milled parts. Maybe there is an auto option but I never tried it.

I was going to add a screenshot but the webpage crashed. I'll edit it in later.