If you select any object in the scene and go into texture paint mode the texture tab will appear by magic and you can then add a new brush texture.
Set the type to none and then open the texture node editor. select Brush as texture type and activate use nodes.
You can exit texture paint mode.
You can now change the nodes (no longer greyed out) and use that brush texture in the compositor as in the tut.
CDMJ was also complaining that texture creation is now bound to brushes I think they managed to persuade the devs to work on enabling access to the old texture nodes directly in compositor but it is very recent and I think not yet implemented. For the moment you have to make a brush.
See this thead (watch this space for progress.)