Eventhough SQL resources are discovered by SnapCenter, underlying filesystems (luns/disks) have to be also discovered.
Under resources, please select 'Filesystem' plug-in and the respective host, trigger a manual 'Refresh'. This operation should discover all the storage foot prints for the disks/luns mapped to the host.
Once this is done, you can recheck the 'log drirectory' configuration. It should list out the disks/luns for you to configure.