You can't search for admin audit log entries because the arbitration mailbox for the organization is located on a server that doesn't have Exchange 2013 installed.

Issue: You can't search for admin audit log entries because the arbitration mailbox

'SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}' for the organization '' is located on a server that doesn't have Exchange 2013 installed. The mailbox must be moved to an Exchange 2013 server before you can search for admin audit log entries.

Resolution: Move arbitration mailbox to Exchange 2013

Get-Mailbox -Arbitration -Identity "SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}" | New-MoveRequest -TargetDatabase <name of Exchange 2013 database>

See reference: http://technet.microsoft.com/en-us/library/dn249849(v=exchg.150).aspx