Can you hit Enter to pause the game, click the 'Item and Skill List' button, and then click on the Dragon Stone from there (near the bottom of the list)? Or assign it to one of the skill slots (pause and Right click at the bottom of a slot).
Where are you in the game? It could be temporarily disabled due to a plot event.
If you head to a waypoint shrine, can you teleport to the battle tower from there (Morgue shrine, or master Chamber shrine)? If you are outside, can you use the dragon morph stone (Q)?
