In the current version of Exchange Online, you create and configure shared mailboxes using the Windows PowerShell command line. You must create the shared mailbox, configure quotas, and then assign permissions so that users can open and send messages from the shared mailbox. To make this task a lot easier, we created a GUI-based tool that you can use to create and configure shared mailboxes.
For instructions about how to install and use the Office 365 Shared Mailbox Tool, check out this wiki article: http://community.office365.com/en-us/w/exchange/1712.aspx (note: in the article its suggested that you use Set-ExecutionPolicy RemoteSigned if the tool doesnt open - it that doesnt help you can try Set-ExecutionPolicy unrestricted)
Get the tool (posted in a .zip file) from the Office 365 Downloads forum here: http://community.office365.com/en-us/f/183/t/45006.aspxe
The tool will make it a lot easier to create Shared Mailboxes. It looks like this: