Vcenter & Esxi 5

Hi

I'm sorry Iasked this questionin the Department of Esxi 5 and I did not find theanswer I think Iaskedinthewrongsection .

So the question is ?

I installed vcenter and when i trying to add 2 hosts Esxi 5.1 to vcenter  Comes amessagethatmust beputlicense  so what  versions need it for Esxi Here .

Regards