I don't have any administrator@vsphere.local user on the vSphere Client, I just login using the root user created during the ESXi installation and there are the defaults users and groups.
I think too that it must be a permission issue probably because I missed some steps as I'm pretty a newcomer to vSphere 5.5 and the Web Client.
The thing here is that I don't use any Domain and I didn't create any user or add permissions; I thought login with the administrator@vsphere.local was enough to make the vCenter appear.