WebDAV is enabled by default, to access your repository via WebDAV you could either provide the WebDAV adress (probably something like http://yourserver:8080/alfresco/webdav/) directly in the Save/Open window or add your Alfresco server as a network place, under My Network Places in Windows, using the same adress as above.
However, when it comes to MS Office it might be better to use the Sharepoint Protocol support, provided that your Alfresco version supports that. The installation documentation for Alfresco Labs 3 provides more info.