Assuming you want to send on channel 1, put this into the Raw MIDI field to set the tempo to 120:
B0 6B 78
Or to set to 140:
B0 6A 01
B0 6B 0C
Here's how it breaks down:
B = control change
0 = channel 1 (numbered from 0)
6A or 6B = controller number 106 or 107 (in hexadecimal)
01 + 0C = 128 + 12 (the desired tempo value, broken into two parts)
In the next app version scheduled for release on Nov. 1, you will have some fields that you can enter control change messages into directly, instead of converting to hex and using the Raw MIDI field, so you might want to wait until then.
Also, I wasn't able to get this to work with my BeatBuddy on the first attempt, but I need to give it another try.