Help: Define Brush is greyed out

[terra]

i have made several custom brushes and now the "define brush" button is greyed out and i can't create any new custom brushes. what happened and how can i get my define brush button to work again?

how can i edit a file of custom brushes and save the changes? every time i try to do this after saving it reverts back to the old file of brushes that includes brushes that i want to delete.