FJ Software

I have several 'profiles' in Outlook. How does MPE know which .pst file I am storing my data in?

Making a semi educated guess; I'd say it uses the file that is marked as the 'Default data store'

Remember though that a Profile is different from a PST data file. A profile is whole environment while a PST is only the data file. Multiple data files can be associated with one Profile. That means that the 'Default data store' changes with every Profile. This possibly could lead to a situation where different Profiles are only partly sync'ed.
But hey, that's just my guess.

In case you use Outlook 32bit and MyPhoneExplorer 1.8.4 you will be able to choose the outlook-profile in the advanced sync settings (its behind the Button "P"). Otherwise the default Outlook profile will be used.