There is a great Ethereum Stack Exchange post that details many different avenues you can take to get ether out of the Withdraw Contract, including a fantastic UI built by the MyEtherWallet.com team. The only thing it lacks currently is screenshots to make using Mist easier.

Once you have the 8.1 Version of Mist (or the Ethereum Wallet) installed and synced you can start the Withdraw process.

Note: The Withdraw Contract only works for accounts. If you are storing your DAO tokens in a wallet contract, you need to first transfer them to a normal account and then the Withdraw Contract will work for you.

1. Watch both The DAO contract and it’s token:

Address: 0xBB9bc244D798123fDe783fCc1C72d3Bb8C189413

JSON: https://raw.githubusercontent.com/slockit/DAO/master/deploy/interface/full.json

2. Watch the Withdraw Contract using the same method:

Address: 0xbf4ed7b27f1d666546e30d74d50d173d20bca754

JSON:

[{"constant":false,"inputs":[],"name":"trusteeWithdraw","outputs":[],"type":"function"},{"constant":false,"inputs":[],"name":"withdraw","outputs":[],"type":"function"},{"constant":true,"inputs":[],"name":"mainDAO","outputs":[{"name":"","type":"address"}],"type":"function"},{"constant":true,"inputs":[],"name":"trustee","outputs":[{"name":"","type":"address"}],"type":"function"}]

3. Go to The DAO’s contract and execute the following Approve function call:

_spender: 0xbf4ed7b27f1d666546e30d74d50d173d20bca754

_amount: 333333333333333333333333333333 (or any number larger than the number of DAO tokens you hold x 10¹⁶… )

Execute from: The address that holds your DAO tokens.

Send ether: 0

4. Go to the Withdraw Contract and execute a Withdraw function call:

Execute from: The address that holds your DAO tokens.

Send ether: 0

After those 4 steps, your ether should be in the address that you used to hold DAO tokens.