ahhh well the boolean commands are the heart of rhino and many cad programs. that is how you join, split and subtract from solid objects. in rhino the boolean union command looks like two spheres half stuck together. click that in the menu and the rest of the booleans should be there. in the mesh drop down menu there will be the same set off booleans but mesh specific. try to not work with meshes. rhino is a nurbs modeler not mesh.