DO NOT bring your part in as an .STL
You open your.bbcd file which has your part drawing in it (not.stl)
To set up your Milling Stock (not your part) R Click on milling stock/stock wizard/check .STL/next browse to where you have the file and select it
On edit,you open your solidworks file,not .bbcd,,,,sorry forgot which software you using,,,but I believe you get the idea ?