You have to finish both the Pyramid of the Sun and the Krokosphinx. When both are done, you should have a message to see Sergeant Major Talbot, who will let you into the Tomb of Storms.
If you've done that, you need to light up each of the three Braziers surrounding the circle where the portal is (near Hetch Al'Dim). Then the Sphinx-like portal will open and let you in. Be aware that it takes about 10-15 seconds after you try to enter it before you'll be let in.
