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).
I'm assuming that you have read some warnings on internet about using this class (e.g.: warning: sun.misc.BASE64Decoder is Sun proprietary API and may be removed in a future release).
It's available in rt.jar, that comes with Sun java runtimes.