After enabling LCR DPM 2007 does not show the First Storage Group which is the one that gets LCRed in Exchange 2007 any more:
That's because DPM can't backup LCR when the VSS writer on that LCR DB is enabled.
So you need to disable the VSS replica Writer on that LCR DB:
You need to add the EnableVssWriter key as a DWORD with a value of 0 (NULL) to the Registry in HKLM/Software/Microsoft/Exchange/Replay/Parameters like shown in the following screenshoot.
After setting this key you need to restart the Exchange Replication Service to activate that setting:
Then you should be able to create a DPM protection group including Exchange LCR Mailbox protection:
This is also referenced here: