Need Guidance

[cyber@tooth]

Hi
I am a hobbyist and am stuck up . I have an appliance which is a “ U” shaped groove . I need to make a positive of it with a couple of grooves for a “O” ring type suction rings.
I had the part scanned and have the positive for it as an .stl file. Which I have imported to solidworks2012 and is saved as a .sldprt file.( Assembly.1)
I have designed the negative for the suction ring and this is also saved as .sldprt file.(Assembly.2)
Please refer to the pictures attached pictures
My problem:-

1. When I go to assembly and open both the .sldprt files the feature manager shows both the files are open.
2. Both the files are fixed
3. I make one file ( suction ring) as float.
4. Unable to see the sketch/model(suction ring)
5. Suction ring designed is in millimeters with a diameter of 3mm while the other part is a of a diameter of 1cm . I have changed the color of the suction ring to dark red but still it is not visible. Can somebody please tell me what Is the issue here.
6. I need to insert the suction ring part into the positive of the scan so as to get a pitted surface which can be 3D printed to obtain a mould/negative die for the part.(Assembly part representation.3)

Can somebody guide me. I have solidworks 2012