This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Actually, you should not be using that class, it is one of Sun's internal unsupported classes. If you need a base64 codec, I recommend the Jakarta Commons Codec API (http://jakarta.apache.org/commons/codec/).
If you really do need that class for some reason, it is most likely in the runtime.jar or one of the other library jars that make up the core JDK (but only in Sun's).