After upgrading from Microsoft Exchange 2003 to Exchange 2007 or 2010 you might notice that the "Enable Web-based distribution" option is greyed out. The first thing to check is if you have moved the Offline Address Book generation to an Exchange 2007/2010 server.
However if you have already done this and the option is still disabled it can easily fixed by running ApplyMandatoryProperties on it. This can be done by running the below powershell command
Get-OfflineAddressBook | Set-OfflineAddressBook -ApplyMandatoryProperties
Et voila! you can now "Enable Web-based distribution"