Hello turki,
If you want to apply the teeth to have a finalized design to mill/print then you can set the case up as a digital copy/waxup (so that it copies your design exactly, and without you having to go through placing teeth). Just be sure that the STL file itself has the bottom of the design where it should form to the margin open (edit mesh and cut out the crown bottom if necessary). Also be sure that the mesh only contains the data that you want to sync to your margin (if there are other existing teeth use edit mesh to cut that off) or else that will also attempt to attach to your margins.
If you're looking to make a diagnostic waxup to show the patient on a solid model, you will need to first create the model, then go to freeform model, and the ATT page. From here you can load the STL file from the bottom of the attachment dropdown menu. There's a possibility that you will need to use the freeform tool to enlarge the preps to intersect with the design to attach it successfully. It is always easier to use this method with single crowns, rather than a bridge. Also be sure that the mesh is a "water tight" one, in Exocad this would mean merging and saving it with the checkbox for "optimize for 3D print" selected.
- Josh
