Short answer : You can't !
MD5 stands for Message Digest
It's a one way
fonction which computes a text's digest.
You can't decrypt a MD5 digest.
You usually use it to check that the digest of the value just entered is equal to the digest stored.
(usually passwd management works this way)
Use other usual encryption
schemes if you want to decrypt text.
I warmly recommend btrott
's impressive work (Crypt::OpenPGP
which handles several schemes
(IDEA, DES3, Blowfish, Rijndael, Rijndael192, Rijndael256, Twofish,DSA, RSA, and ElGamal)