Come to Computer Services during normal operating hours. See the Lab Manager, Chas Moody. She will have you fill out and sign a contract.
The contract gives computer services the right to revoke your after hours priviledges if you are found breaking the rules for use (the rules are listed in the contract and you will be given a copy of them).
Once you have signed the contract, you will be given the top portion of it (which lists the rules) with the door's combination written on it.
