Not sure what you already know, so I may be giving too much or not enough info, but ...
All Concerns will appear as the same type. So if one Concern is Impulsive Response, the others will be Playing a Role, How Things Are Changing, and Doing. If you put your OS in Activity and then choose any of the other Concerns I just mentioned, or even any of the items under any of those Concers, it should force your OS Concern into Doing. This has to do with making sure all four problems are looking at the same kind of problem from a different angle.
There are a couple of other ways to force Doing as well, but they're not as easy to explain. For instance, if you choose MC Issue of Value (or anything under Impulsive Response) and a growth of Start, your OS Throughline will be put in Activity and the Concern in Doing. This has to do with how Growth is determined along with making sure all the Concerns match.
Dramatica is about giving you the process the human mind follows in order to solve a problem. Just as above, depending on what info you put into Dramatica, you will eventually reach a point that you force the story into Change or Steadfast. I'm not as clear here, but I presume the reason why it forces you into Change is because the only way to progress through the rest of the path you've chosen is for your character to Change.
Again, hard to explain in terms of Dramatica theory, but think of it like this. If your character is driving a car straight for a cliff with the goal of not going over the cliff, then in order to succeed he will be forced to change the direction the car is moving in, or even stop the car. He has no other choice because if he doesn't change, he will fail to reach his goal of not going over the cliff.
You MC is a Do-er because s/he sees the problem as an external one. You've done something to put them into a problematic Situation or Activity. When your character sees the problem as external, they prefer to try to change the external to solve the problem rather than to make an internal change to try to solve the problem.