Adrian is correct. The Tempo tool, which was just another interface to the MIDI Tool Tempo editing, was removed in v25. You can set the initial tempo with an expression and define the specific tempo in the Playback pane of Tempo Mark Definition. Human Playback will also detect text in your file ilke "rit." or "accel." to adjust the tempo in a natural way. If you need further control you can use the MIDI Tool or you could even tap in the tempo with the Tempo Tap Staff found in Studio view.
General article concerning Tempo with more details than what I've described can be found here for Mac or Win
Cheers, Michael Johnson VP, Professional Notation MakeMusic