Do you want to cut a hole? or a disc?
Create a sketch, and draw a circle. Then use Extrude Boss to make a cylinder. Then, create a drawing of the cylinder, with a scale of 1:1, starting with a blank page. Export the drawing as a .dxf, and load into CAMBAM.
Or, download a free 2D CAD program like A9CAD, draw a circle and save as .dxf.
For simple 2.5D stuff, IMO, Alibre is probably not the best choice, due to the extra steps involved to get a 2D .dxf. You need to start with a 2D sketch, then use that to create a 3D model, then create a drawing to get back to 2D.
Also, if you're using Mach3, you can use the Cut Circle wizard real quick.
Never tried CAMBAM, so can't help any there. Good Luck.