With SimplyTweet 3.2, you can now send messages to people on your address book. To set this up in SimplyTweet, press the avatar of the account to reveal the account view.

Press the address book button at the top of the account view. This will bring you to the screen where you can, either create a new contact in your address book, or add to an existing contact in your address book.

Choose one of them to complete the set up. The next time you want to send a direct message to this account, just open up the contact in your address book and tap the "Twitter DM" field.