With the advent of Exchange 2010, Microsoft dropped WebDAV support, switching to Exchange Web Services (EWS). This means that older versions of Entourage are not compatible with Exchange 2010.
You need to download an EWS capable version of Entourage, available here:
From the Entourage Menu select 'Account Settings'.
If the Account Setup Assistant pops up, enter your e-mail address, tick the 'My account is on an Exchange server' and then select 'Configure Account Manually' option. Otherwise, select 'Exchange' from the drop-down box and select 'OK'.
Enter the following in the 'Account Settings' tab:
Name: (Your name)
E-mail Address: (Your Exchange mailbox address)
Account ID: (Your Exchange mailbox address)
Password: (Your mailbox password)
In the 'Advanced' Tab:
Exchange Server: https://cluster1.exchange.outitgoes.com/EWS/Exchange.asmx
*Important* If Entourage pops up, asking you whether you want to automatically set up the account and receive settings from the server, make sure that you select 'No' - or it may stop working. It will attempt to set up an LDAP server, which won't work.