Welcome to the Community,
I assume you are installing ESXi on a Whitebox, i.e. not officially supported system? The reason you cannot see the drive/datastore is most likely due to an unsupported controller in your system (see http://www.vmware.com/go/hcl). In some cases it's possible to make it work with changing BIOS settings.
What you could do is to check what type of controller is detected in ESXi 5.1 and check whether there's a driver available for 5.0 too.
André