Authentication problem if CHAP is not used

Hi, I have a problem; I always get Access Reject saying that password is invalid. But when I'm using NTRadPing Test Utility, the same username and password works fine if I check to use CHAP, but when the CHAP check box is not selected then I'm getting the same problem as I have with my NAS. So can some one tell me how to make FreeRadius work so that if I'm using NTRadPing without CHAP it would still work! Thanks!!!