According to the error message ("Could not find or load main class"), there are two categories of problems:
Main class could not be found
Main class could not be loaded (this case is not fully discussed in the accepted answer)
Main class could not be found when there is typo or wrong syntax in the fully qualified class name or it does not exist in the provided classpath.
Main class could not be loaded when the class cannot be initiated, typically the main class extends another class and that class does not exist in the provided classpath.
public class YourMain extends org.apache.camel.spring.Main
If camel-spring is not included, this error will be reported.