I am not using Keyboard Maestro in a midi setup but with that said I only see 3 midi actions. The most useful for most cases would be the send midi control change. I think you probably want to start with that and designate one of the channels (I use channel 15 for control changes on my guitar rig) as a controller channel, (depending on what type of information is being controlled).
The best thing to do is:
Describe the workflow you wish to accomplish and,
Post your current macro along with,
A screenshot of all the steps.
I will try my best to help in any way I can and I know there are better minds here who most definitely can help.
What controller are you using? What are you trying to do? If you can explain what the scenario is, what equipment you are using and what you are trying to accomplish it would help.
I am also curious why you wouldn’t just use a controller that has a start/stop or a start and a stop button? So many controllers out there that have dedicated buttons for this and many others that you can program to send what ever you could dream up.