It's actually simple (not logical but stil simple).
Go to adresbook preferences and
enable Synchronise with Exchange type in username password and server.
Web acces shoul be enabled on the exchange server.
Download and install a new ("old") iSync as the one supplied with the beta has expired ;-).
Open isync and you will see an Exchange icon, wich looks like a blue adressbook with a windows in it.
Because Mail does not show the Calendar folders when connected via iMap to an Exchange server, my guess is that apple is going to incorporate a nice interface to the other side ;-).
If you choose normal imap the calendar folder does show.