It is definitely a classpath issue. You say you "converted" the ZIP to a JAR file. Did you simply rename it (which is the correct thing to do), or did you do something else? If you did something else, you might want to open the JAR file and make sure the classes and paths (packages) are OK.
If everything in the JAR file seems to be OK, then it means your classpath is set wrong. That what the error means.
As a secondary point,
you should NOT include the "DriverManger.registerDriver(...)" call in your code; it will be done for you as part of the "Class.forName(...)".