this is by design in scom 2007 r2. a few simple steps will get you there though.
verify discovery helpers
-don't scope your views and click on "discovered inventory" in the monitoring pane
-right-click and click "choose target type"
-type "Exchange 2007 Discovery Helper" in the look for field
-exchange 2007 servers should appear w/ "not monitored" status
enable the discoveries (see note2 and image below for more info)- http://blogs.technet.com/momteam/archive/2010/02/23/exchange-2007-management-packs-do-not-automatically-monitor-exchange-servers.aspx
note2: hub transport and client-access are easy to find in the list of object discoveries. mailbox role is under the rule named "Exchange 2007 Standalone CCR Node Discovery". everything else is pretty self-explanatory.