Use the B snap to rough set them (there is a tiny space in them), then afterwards, left mouse button on them and arrow them over into place.
For object to be able to snap to each other perfect, they must have the same shaped sides.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit