All the fonts are added to FontBase in an inactive state. To activate a font, you need to click on the circle at the left side of the font's name. Most of the apps (Photoshop, Sketch and other) should recognize the activation immediately and display the newly activated fonts in the font list right after the activation. If your app for some reason does not display the activated font immediately, try relaunching it.
All the fonts activated in FontBase are active while FontBase is open. When you close the app, all the fonts are deactivated: this way you can easily deactivate all the fonts you have used during your work. When you launch FontBase the next time, all the fonts that were active before you closed the app will be re-activated again and you can start using them again.
Please note that activated fonts do not stay active when FontBase is shut down or after rebooting the computer.