I am currently working on my first storyform with a Steadfast Main Character and I don't understand one thing: If my Main Character is steadfast what does Solution mean for him??

When you have a Steadfast Main Character (MC), it's best to see his "Problem" as the source of his drive—his motivation. The Solution is the thing that saps him of his drive by removing the motivation. Since he’s a steadfast character, the solution is not adopted by the Main Character and his “Problem” continues to drive him.

A Steadfast Main Character Symptom is the thing the Main Character THINKS is the source of conflict in his personal life. The MC Response is what he thinks is necessary to address conflict created by the MC Response. A steadfast main character works to resolve conflict in his personal life by treating the symptoms, not the problem. Though he may be tempted to address the problem by adopting the solution he never does—not doing so is what makes him steadfast.

Here’s a Main Character Problem/Symptom analogy: Imagine the MC Problem is a disease and the MC Solution is the cure for the disease. As deadly or dangerous as the disease may be, it might also be hidden from detection. However, imagine the MC Symptom as the symptom of the disease and the MC Response as the treatment for that symptom. In some cases, treating the symptoms will not prevent the problem from getting worse and only curing the disease removes the problem (Change Main Character). In other diseases, there is no cure but treating the symptoms can be sufficient enough to survive (Steadfast Main Character).

Whether the Main Character makes the right or wrong move by treating the symptom instead of the problem is determined by the Story Judgment (Good or Bad).

There is a short discussion of this topic towards the end of the podcast of this April 2006's Dramatica User's Group meeting In the Heat of the Night that goes into a little more detail. I've marked the podcast with chapters so that you can forward to the various topics faster.