Before you can send a tweet or DM, you must have connected your Twitter account to SharedBox.
Type /tweet in the message box of the channel the Twitter account is connected to and press enter. This will show you a dialog box you can type your tweets in.
You can also type your tweet in the format /tweet Tweet to post here to pre-fill the tweet body in the dialog box already.
Sending a DM is similar to sending a tweet. Type /twitterdm username and press enter. This shows a dialog box you can complete and send.