My machine came with 3D Embroidery. It uses the same process, marjialexa describes, except my program requires me to only click on the "handle" (circles, squares, or triangles around the design)that is at the top and center of the design. If I use the other "handles" located anywhere else on the design, then the design is rotated or resized. Hope this helps.
I use Janome Digitizer Pro. When I add the second design, it puts it in the middle over top of the first one. When it appears, there are the little squares bounding the design, as if you were adding a photo. If you put your pointer inside the squares and hold down, you can move the design anywhere you want in the field. It's just like moving a photo in Publisher or one of those programs. That's how mine works, anyway, hard to imagine others would be all that different. If you need any more help, just ask, and let me know what program you are using. If it's not Janome, there are probably others out there using the same program who could help you. Best of luck to you, and good luck on the Carnival fonts, they're great, aren't they!! Marji