By default, local storage devices are disabled as potential RDM candidates on an ESXi/ESX host. To enable it, you need to perform the following set of actions (having a vm named "w7" stored on the "ds" datastore).

SSH Host
[cc lang='bash']
# get the disk you want to be presented as a Raw Device (the one with no partitions configured - S1DA7CKC)
\$ ls /vmfs/devices/disks/

t10.ATA_____ST1000VX0002D1CU162__________________________________S1DA4TCN

t10.ATA_____ST1000VX0002D1CU162__________________________________S1DA4TCN:1

t10.ATA_____ST1000VX0002D1CU162__________________________________S1DA7CKC

t10.ATA_____ST3500411SV_________________________________________Z2ASKZZQ

t10.ATA_____ST3500411SV_________________________________________Z2ASKZZQ:1

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:1

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:2

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:3

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:5

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:6

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:7

t10.ATA_____ST3500411SV_________________________________________Z2ASL00N:8
vml.0100000000202020202020202020202020533144413454434e535431303030
vml.0100000000202020202020202020202020533144413454434e535431303030:1
vml.01000000002020202020202020202020205331444137434b43535431303030
vml.01000000002020202020202020202020205a3241534b5a5a51535433353030
vml.01000000002020202020202020202020205a3241534b5a5a51535433353030:1
vml.01000000002020202020202020202020205a3241534c30304e535433353030
vml.01000000002020202020202020202020205a3241534c30304e535433353030:1
vml.01000000002020202020202020202020205a3241534c30304e535433353030:2
vml.01000000002020202020202020202020205a3241534c30304e535433353030:3
vml.01000000002020202020202020202020205a3241534c30304e535433353030:5
vml.01000000002020202020202020202020205a3241534c30304e535433353030:6
vml.01000000002020202020202020202020205a3241534c30304e535433353030:7
vml.01000000002020202020202020202020205a3241534c30304e535433353030:8

# create a RDM passthru for that disk
\$ vmkfstools -z /vmfs/devices/disks/t10.ATA_____ST1000VX0002D1CU162__________________________________S1DA7CKC /vmfs/volumes/ds/w7/local-rdm.vmdk
[/cc]

vSphere
[cc]
right-click the "w7" vm
click "Edit Settings"
click "Add"
select "Hard Disk"
select "Use an existing virtual disk"
browse to "ds/w7/local-rdm.vmdk" and click "Next"
click "Next"
click "Finish"
[/cc]

You should now see your new hard disk in the virtual machine inventory as Mapped Raw LUN.