BotMixer requires MapMixer in order to function. Check the History section to make sure you have the correct version of MapMixer.
For each match, BotMixer is prompted by MapMixer to select bots. To activate BotMixer you must enter the MapMixer Configuration Menu and Enable BotMixer in the Player Counts section.
BotMixer contains its own Configuration Menu that is opened by entering the MapMixer Config Menu and clicking on the "BotMixer" button next to the checkbox where BotMixer is enabled.