Hello,
I was able to configure Sharepoint 2013/Exchange 2013 (both on Windows Server 2012 R2) for Site Mailbox and followed directions from several well respected articles and from the Technet: https://technet.microsoft.com/en-us/library/jj552524.aspx
Initially, I had issues with SharePoint locating the GAC but that that was fixed by removing and reinstalling EwsManagedApi.msi. All the scripts mentioned below ran successfully.
Check-SiteMailboxConfig.ps1
Set-SiteMailboxConfig.ps1
Configure-EnterprisePartnerApplication.ps1
My Farm is one FE server and one SQL server. I was able to add a site Mailbox in a Team Site. And when I clicked on the APP, it redirected me to OWA successfully. (it prompted me for credentials. not sure if SSO is in effect here) Users in the Team Site are all Owners. Unfortunately, nobody including myself sees the Site Mailbox in Outlook client.
We are using Office 2013. The Site Mailbox does not show up when we login into our OWA. We also don't see "manage site mailboxes" when we right click on our Inbox folder via Outlook Client. When we login to our Account in OWA and choose Options, we see that we have permissions to Access the Site Mailbox. We started the Sync manually and it was successful. But still can't see the Site Mailbox in OWA or Client. Scoured through the blogs, forums, microsoft but nothing. An suggestions is much appreciated.