MD5 isn't an encryption algorithm, but a one-way cryptographic hash function. You can't decrypt the resulting hash value.
For encryption / decryption you'd need a symmetric key algorithm like DES, which you also mentioned in your starting post.
The Java Cryptograhpy Extension reference guide should provide you with the information you need to get going.
Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.