BotMixer is an optional addon package for MapMixer that handles Bot Lists (called Teams). It can be downloaded from the MapMixer website.
BotMixer's main purpose is to manage custom bot selections. If you are using a custom map list in MapMixer and any of these maps contain customised bot selections (ie Map Edit) then these will override BotMixer's choice.
If BotMixer has been installed then a CheckBox will be visible in the Player Counts section on the Setup page of the MapMixer Config. This CheckBox will enable BotMixer and you can configure BotMixer by clicking on the "BotMixer" button to the right of it.
See the BotMixer Documentation for a full description of BotMixer's features.