In Mixere, tempo is a scaling factor that's applied to automation times. Tempo only applies to automation sliders in tracks; other automation sliders, e.g. document volume, aren't affected. The scaling factor defaults to 1 (no scaling), and ranges from 0.5 (half time) to 2 (double time). Tempo shrinks or stretches the time axis, i.e. it changes the duration of a second, so that that a one-second automation actually takes as little as half a second, or as much as two seconds.
Each document has its own tempo, which can be changed using the Tempo control bar. To show or hide the control bar, choose Tempo from the View menu, or press T. The control bar includes a tempo slider, and an edit box for entering specific tempos. Note that the tempo slider itself supports automation, which means a document can be programmed to gradually speed up or slow down its automations. This is useful when samples are being played in a synchronized pattern, using Auto-Trigger; the effect is similar to creating a tempo map in a sequencer.
Tempo is displayed in beats per minute (BPM), according to tradition, but since Mixere doesn't support notes or time signatures, the definition of a beat is arbitrary. In Mixere, a beat is a half-second auto-trigger loop, which triggers twice per second, or 120 times per minute. To demonstrate this, load an audio file containing a single percussive sample, and auto-trigger the sample with an automation time of 0:0:0.50, while Mixere's tempo is set to 120; the resulting loop should stay in sync with a metronome set to 120 BPM.