Another solution would be to perform the following:
Step 1) Make sure remote tech support (SSH) is enabled and running.
Step 2) SSH to the host
Step 3) fdisk -l | grep -B4 'doesn't contain a valid partition table'
Note: This will show you all the physical disks that don't have partitions yet, such as a newly-provisioned SAN LUN. It should look something like this:
Disk /dev/disks/naa.60060e801004eb90052fab6900000000: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/disks/naa.60060e801004eb90052fab6900000000 doesn't contain a valid partition table
Disk /dev/disks/naa.60060e801004eb90052fab6900000001: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/disks/naa.60060e801004eb90052fab6900000001 doesn't contain a valid partition table
Disk /dev/disks/naa.60060e801004eb90052fab6900000002: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/disks/naa.60060e801004eb90052fab6900000002 doesn't contain a valid partition table
If this command doesn't show you any devices, my procedure probably isn't for you, as I, like the previous posters, make the assumption that the reason your VC "Raw Device Mappings" radio button is greyed-out is because the LUN doesn't have a partition.
Step 4) Create a new partition, "fdisk /dev/disks/naa.60060e801004eb90052fab6900000000"
Note: You'll have to use your own device name here.
Step 5) If you're not too familiar with fdisk, you can do this:
a) "p" to print existing partitions. If you don't see any, then it's probably safe to proceed.
b) "n" to create a new partition.
c) "p" for primary
d) "1" for partition 1
e) to select default start sector
f) to select default end sector
g) "w" to write
h) "q" to quit
Step 6) Now you should be able to assign the raw disk in VirtualCenter.