Temporarily switch to Blender Render (if you’re not using it anyway).
Create your texture in the Texture tab.
In the compositing node editor, hit Shift-A > Input > Texture.
Select the texture you just created in the new node’s dropdown list.
It seems unintuitive to have to go to the Blender render to set up generated textures for Cycles, but I suppose, maybe, that is just something that a programmer has to decide to invest the time to fix.