You probably see this rare problem: your storage team allocate new lun to esxi 5.0 host, lun is visible in add new storage screen, but invisible in extend datastore  screen.

Add new storage screen:

Increase datastore capacity:

 

That’s because the datastore, lun is connected to multiple esxi / esx host which have different version, please be sure storage is connected to same version of esxi / esx host.