Hmm... this suggests that Eclipse is confused about how to run your program -- probably something is not in the right place where Eclipse expects it to be.
One possibility is that you might not have put your .java file inside of the src folder, within your project. (If that's the case, then move your .java file into the src folder).
Another possibility might be if you created a generic "project" instead of a "Java project" inside of Eclipse. (If that's the case, you can create a *new Java project, and drag your file into that project's src folder instead.