I am having trouble finding the Sequencer button. I have BIAB for Mac 2018 I see no Sequencer button, even when I use the arrow to expand the "Tools" window.

I'll see if I can append an image to this post -- new to forum, still figuring out how to do that.

The sequencer window is not present on the Mac version currently. However you might not need it to do what you want. Mainly it just allows you to easily solo/mute individual tracks if you import a multi-channel MIDI file to the Melody or Soloist track. What are you trying to accomplish?