I can't help you with Mastercam, but generally speaking, lathe paths are simple 2d chains. Once you figure out how the screen coordinates jive with the real world setup of your lathe's axis, it is simply a matter of drawing in the correct quadrant.
For a slant bed lathe, with tool approach from the rear, it seems logical to draw the path in the Z-X+ quadrant. For a conventional lathe, you still have to draw in this quadrant, but most likely, this will seem opposite to the layout of your machine.
The part's outermost face is usually drawn to coincide with the X axis, at Z0 in other words. All cutting takes place in the Z- or X-direction (typically). Of course, the part's axis of rotation coincides with the Z screen axis, or X0.
It is not necessary to draw a complete "double-sided" cross-section of your part, since you only need to construct a 2d toolpath along one side of it.
Is that of any assistance?