While I don't have any specific experience with the Verisign API (not even sure what you're referring to), the class not found indicates that your classpath can't find the class (or package) com.Verisign. You'll need to either get the class, or more likely a jar file containing the class. I would check with the Verisign installation documents.
BY API I meant the predefined methods or logic from verisign. I was also thinking of finding the versign.jar or some other jar. I was asking whether we can download it or it should be provided from versign with the licence?