Because Citrix has decided to give away the XenServer 5 for free you will need to do some updates to get the new license file. First of all install Update 3 for XenServer & XenCenter and then remove your old license file. Citrix has posted the CTX120960 article about these issues.
The console now shows the new license file which you have to activated with tween 29 days. Register online and then install the new activated license file.