The issue is that the preinstaller is not sure which mailbox db to put the service account.
There are two ways to solve the problem:
Make the account manually in the mailbox you want manually and then
Log in to one of the Exchange Servers (with Exchange Command Shell installed) as the domain admin
run the Preinstaller
and check that the account already exists.
OR
Log in as the domain admin
run the Preinstaller
Click on OU button and open the text file
copy the appropriate ou in the dialog from the text file to the preinstaller
Then click on the MDBs button for the Mailbox DB and open the text file
copy the appropriate MDB in the dialog from the text file to the preinstaller Mailbox DB field
Then do not check that the account already exists, but type in the password twice (if you make a mistake on the password, you have to close the Preinstaller and start over. The next time, just copy the appropriate info into the fields. You don't have to push the button again, just copy to info to the two places)
Then run the preinstaller again by pressing OK.
That should do it. Be sure to make the A2E service account part of local administrators of all exchange servers we are replicating to or from in order to test to make sure the exchange servers are up during sync.
That should do it. If you need further support, please fill this out: http://support.diditbetter.com/support-request.aspx
Hope this helps