This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
It means you have no .class file matching the name in your current directory. Go and find the .java file for the class with the main method it, and read its package declaration. If there is no package declaration, then you need to navigate to whichever directory contains that .java file, and check there is a corresponding .class file with the ls command.
If there is a package declaration, you ought not to be in the same directory as the .class file. If the .java file starts like thisyou should be in a directory which contains the directory called "com"; "com" contains "foo" and "foo" contains "bar" and the "bar" directory contains MyMainClass.class. At least it will if it has compiled correctly. You invoke the main method by writing
java com.foo.bar.MyMainClass args0 args1
Try that, and make sure all the .class files are in the right locations.
That error has nothing to do with the PATH variable.