If you add a custom brush to a custom interface, that brush will not show up on the interface until it is loaded into the program. Zbrush has a default list of brushes that load on startup. The rest of the brushes are typically stored in the Lightbox directory to be accessed on demand. Once loaded though Lightbox, you would see it pop-in on your interface, as long as everything else was in order. In order to get a brush to load alongside a custom interface, that brush must also be added to the Zbrush startup list in Zbrush4r7/ ZData/ Brushpresets (I think).
This isn’t something you want to do casually, though. Loading too many extra items into the default Zbrush startup list will begin to affect the program’s performance, and I believe there is a hardcap though I can never remember what it is. It should be reserved for items you will be using all the time. More situational items should be stored in the Lightbox directories, and recalled on demand. Remember you can make those directories be whatever you want, and even store custom items in the root to avoid having to click through too many folders.