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 guideshould 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.