The brush selector is disabled when you don’t have the brush tool selected. This took a while and quite a bit of stabbing the selector before I figured it out.
Instead it would i.m.o. be reasonable to assume that when you select a brush you want to use the brush tool, so the selector can be active when you use select and/or deform tools for example, and when you select a brush you get automatically switched to brush mode.
What do you think?