As I understand it, the MC Problem is what's "wrong" with the MC.

In a story where the hero Changes and the outcome is Success/Good, this is the character flaw the MC has to overcome in order to solve the story problem. In a story where the hero is Steadfast and the outcome is Success/Good, this is what everybody else thinks the MC is doing wrong. In reality the MC Problem is what allowed him to overcome the story problem. (For example, I think Dirty Harry or Jack Bauer's MC Problem could be that they're willing to break the rules to get results.)

Is this right?

Not quite. You've confused a few different story points.

Success is tied to the Story Outcome and determines whether or not the story goal has been achieved.

Good has to do with the Story Judgment and is related to whether or not the story inequity was resolved, as opposed to the MC Problem.

Change has to do with the Main Character Resolve and indicates that the MC has adopted the MC Solution.

Each of these are independent, which means you can have any combination of Outcome, Judgment,and MC Resolve. The audience may infer connections between these because each has part of the meaning in the story, but the CAUSALITY is non-obvious and non-linear.

For Steadfast MCs, it's best to think of the MC Problem as the source of the MC's drive. What other characters think it is may not be relevant and is not relevant to its functionality in the story development.

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.