The push pin or end that looks like an arrow is pushed in from the bottom. The round "T" head fits into that key hole shaped slot at the top rear or the case. Feed the "T" into the hole and gently push the MB to the rear of the case and the rest of the screw holes should line up with the metal stand offs
Do recheck on that one screw in stand off for the back of the case by the top PCI slot, I believe you will need it... every ATX or mATM MB I've ever seen requires it.
Also, those little square holes in the case, there may be plastic pop in towers that will appear square like a box shape. That type is also used in some case when additional stand offs are needed.
They look like...
BTW, is all this all about the MB in your signature?