I created a program on NetBeans. The package contains three files: MainProject.java, Input.java, and Output.java. When I run the MainProject.java file on JCreator, it opens up the Input.java form, but when I click the "finished" button on the Input form, it doesn't open the Output form. How do I fix this?

Not enough info to help you with your forms, but for moving NetBeans to JCreator I would say it is hard task as JCreator is very simple editor and has different way of organizing project. Unlike advanced/professional IDEs it hasn't got project import option.
So either stay with NetBeans or start new project in JCreator and manually copy classes.