Since this is a homework assignment, the best thing that you can do is ask your instructor for input. The person grading you has expectations, just like in the workplace your coworkers, managers, and customers have expectations. The expectations need to be defined by the person receiving the work.
That said, I can provide my take on how to structure a use case diagram for this particular situation.
The first thing that I would do is to identify the external-facing operations that your simulator system exposes. The text of your question identifies a few - load a configuration file, run the simulation, view statistics. Are there others? Can you pause simulations? Or adjust configuration in the middle of a simulation? Or export the statistics to various formats? Anything else that your system exposes to a user or outside system should be given a use case "bubble".
Next, I would define relationships between the use cases. The three relationships that appear on a use case diagram are extends, includes, and inherits. The extend relationship allows one use case to continue beyond the base use case. If Use Case B extends Use Case A, then all of the steps in Use Case A are completed prior to executing Use Case B. The includes relationship captures dependencies and allows behavior from one use case to be included in another. If Use Case B includes Use Case A, then the operation of Use Case A is available as part of Use Case B but does not state where during the execution of Use Case B it is used or if it is required or not. Finally, inheritance allows for a use case that inherits some of the operations of a different use case but changes or adds additional steps.
Third, I'd identify all of the actors on the use case. These would be human users or external systems that interact with the system under design. It may be wise to consider roles. Humans of different roles may have different access to functions.
Finally, I'd identify relationships between actors. One actor may be a superset of another actor, and this can be indicated on the diagram, making it cleaner and easier to read.
Scott Ambler has a good article on the use of use case diagrams and an article on reuse in use case diagrams. I find much of his work helpful when considering modeling and documenting software systems.
After all of that, I would have to recommend once again going to the person who will be using your use case diagram - in this case, the instructor grading you - to find out exactly what they need from you. In UML Distilled, Martin Fowler almost discourages the use of Use Case Diagrams in favor of other tabular or textual representations of use cases. Diagrams don't have as much value as other more detailed descriptions. Use case diagrams aren't things that you tend to see or use in industry, but you should be aware of their existence and how to read and create them should you be asked to do so.