How to force an alignment between the cad model and the scanned workpiece?

Create a plane on the surface you want to use in the alignment, then align using the created plane.

How? When I click on the alignment button it appears this window (see the attachment)... I can't select any created plane but I can only click on the automatic button or I can take manually the points on the CAD model and the scanned piece.

This is the point cloud alignment utility. Go to Insert, alignment, new....

Actually, the desired alignment is between the CAD model and a point cloud. The workpiece is a scan obtained by the arm scanner. By doing the alignment through the "new alignment" it doesn't work. It changes only the position and orientation of the origin on the CAD model but not how the CAD and the cloud point match each other.

Using the "new alignment" does not change the position and orientation of the origin on the CAD model. The position and orientation of the model origin is set either in an external CAD software such as SolidWorks or via the transform function within PC DMIS. I recommend using an external CAD software to do that.

The "new alignment" changes the position and orientation of the trihedron. The whole concept of the alignment process is to make the position and orientation of the trihedron match the CAD origin. Once you do that, the COP will overlay the model. Once in a while you may have to use "CAD=PART", just make sure you save the program before doing this as it can not be undone.

In your case if you are just concerned about getting the plane to "match" the model. Use the point cloud alignment tool that you have pictured above and do a best fit. Then go back and do an auto plane on the desired plane and use the "CAD offset" to filter out the points around the edge, so you are not picking up any fillets or anything like that. Use the "new alignment" tool to level and origin to the plane, just in the direction perpendicular to the plane. Now translate back the nominal CAD distance to put the trihedron back at the CAD origin. This is only a partial alignment as you only constrained 3 degrees of freedom but it will accomplish what you are trying to do.

You will probably have to do a CAD=PART command once you translate back to the CAD origin as this is only a partial alignment. Just make sure you save the program first before you do the CAD=PART command, just in case it does some thing weird. As said above, CAD=PART can not be undone.