To enter the Altar you have to have at least one rune essence in your inventory, you have to right click any of the wool blocks on the rock and you will be teleported to the area pictured below. Then you have to right click the altar to open up a GUI and press the rune to craft them. To exit the altar area you have to step into a corner of quartz which will send you back up.
It has now been moved to RS-south of Falador, near the Crafting Guild.
The coordinates are
X: -903Z: 472