The simple answer is you can't, you're not ment to be able to unencrypt an md5 hash.
You simply compare a string to the MD5 hash you have stored.
For example, you have usernames and passwords stored in a database, a user enters a password, you change this to an MD5 hash and compare the result with the stored MD5 hash. If they match, the password is correct.
The complex answer,
As with everything, nothing is perfect. You can go about brute forceing the MD5 hash by entering a combenation of letters and numbers until the 2 hashs match giving you the password. Or the much quicker way, use a premade rainbow table to lookup the hash and find the word it relates to.
This can be made harder to crack by using a seed while generating your stored MD5 hash.