Igor, the 5131 is only a WiNG-4 AP. There are *no* versions of WiNG-5 that operate on a 5131.
If you already have a network in place using the RFS6000 with other WiNG-5 APs, and you want to also use the 5131's, you would either need to setup another controller using WiNG-4 so that it can mange the 5131's, or you can setup the 5131's to operate autonomously, without a controller.