Exchange 2007 offline address book error

I get the following error for my Exchange 2007 offline address book:-

“OALGen is configured to generate version 4 offline address book (OAB) files for OAB ‘/o=The Enterprise Department/cn=addrlists/cn=oabs/cn=Default Offline Address List’ and publish the version 4 files to public folders, but there is no public folder server available. OAB version 4 will be generated, but will not be published to a public folder at this time. Please ensure that a public folder server with a replica of the OAB system folder is online and mounted, or disable publishing OAB version 4 to public folders.”

Any ideas how to solve it?
Simon.