Basically, in Outlook (and even eM Client, which, in my humble opinion, is WAY better than Outlook) you have to set up delegation in the client to open the shared account. 

In Outlook for Mac, that entails going to your account, clicking the Advanced button, and selecting the Delegates tab. Here, you can add the shared inbox to "Open these additional mailboxes:" by clicking the plus sign (+) and typing in the mailbox address (or just the username portion). Outlook should discover the share and allow you to add the inbox. 

Here's how my Accounts look, where "Device Management" is a shared Inbox: