Hi Gizmo - generally you use the Fillet tool to round off sharp edges where pieces meet.

I'd recommend viewing the video tutorials here:
http://moi3d.com/1.0/docs/tutorials.htm - especially the sunburst and 6-legged pod one, both of those demonstrate using fillets to round off sharp edges after you have done booleans to combine pieces.

Here are some steps for one way of how you could approach this.

Start by drawing a sphere and then some curves like so:



Select the sphere, run Edit/Trim and pick the curves as cutting objects, then select the top and bottom pieces as pieces to discard, that will create this:



Some steps for filleting can be a little easier if you work with solids instead of surfaces, so to make some of the later steps a bit more streamlined, you can use Construct / Offset / Shell to thicken this trimmed surface into a solid:



Now draw a circle off to the side:



Run Construct / Sweep and pick this edge as the rail curve (if you trimmed using multiple curves instead of a single curve, then you'll need to join those edges together into a single new long curve before sweeping):



The sweep will then create this:



Now you can use Construct / Boolean / Union to merge these 2 solids together. There will be a new edge formed where they intersected each other:



You can now select that edge and run Construct / Fillet on it:



Here is the final result being exported to polygons:



Hope that helps get you going!

- Michael