Yes, Edit > Locate about origin is the correct command to center an object in Anim8or's Cartesian coordinate system. Be careful, though, because it calculates the center of the object based on its geometry. Hence, a triangle (or pyramid) won't actually be centered based on its actual centroid. That's not a problem for a sphere, naturally, since its centroid is equidistant to all points on its surface.
A 3-dimensional Cartesian coordinate system references everything to the origin, the location where the X-, Y, and Z-axes intersect. And yes, the origin is located at 0, 0, 0, and everything is measured in plus or minus directions along those cardinal axes. It really helps if you have a fundamental understanding of this concept, and geometry and trigionometry can help a lot, too!