Select the verts of the object you want to mirror. The go to the edit menu (like on here) and go to the Object editor (it will have an O next to it). Then go to the second option (has an M next to it) This will COPY and MIRROR the object you have selected. If you dont want to keep the original, I suggest pressing Shift+Delete to delete the old object (You will still have the old one selected, so you wont delete the new one you just made). All done!