I’m assuming you are using the local drives to your server to host your datastore? Did you partition the free space with VMFS during installation? If you didn’t then you will need to first do a scan to confirm it can see the drive.
Within the VI Client, do the following:
1) highlight your esx server
2) go to the configuration tab
3) under hardware, highlight “storage adapters”
4) highlight vmhba0 (type Block SCSI)
5) under details, you should see all your local drives that your server would see.
6) now go to storage
7) go to “Add Storage”
8) use the wizard to select local drive. Make sure you select “free space” and not “all partition” since it will delete your ESX installation.
9) once you are done, you should now see your local space.