If you find yourself having to debug the Exch2010Extension.dll in the FIM Sync 'Extensions' folder, then you want to attach to the MMSSCRPT.exe process, and not MIIServer.exe.
2015-04-30 07:21:52,196 --1-- DEBUG [Exch2010Extension.Exch2010ExtensionClass]
Process information
-------------------
ID 3704
Name mmsscrpt
ToString() System.Diagnostics.Process (mmsscrpt)
CPU time
Total 00:00:02.4843750
User 00:00:02.0156250
Privileged 00:00:00.4687500
Memory usage
Current 54,919,168 B
Peak 54,919,168 B
Active threads 21