Hi Kerim.
I would need more information to be able to assist with this. Can you please answer the following:
1. Step-by-step, how are you trying to add the switches?
2. Are you seeing any error messages...if so...what is the message?
3. Have you verified credentials? What are the results?
4. (bonus) can you *manually* telnet/ssh to the switch and snmp poll it from the NCS?
Thanks,
-Joe