I have never dealt with nltk but did N-grams as part of some small class project. If you want to find the frequency of all N-grams occurring in the string, here is a way to do that. D would give you the histogram of your N-words.
D = dict()string = 'whatever string...'strparts = string.split()for i in range(len(strparts)-N): # N-grams try: D[tuple(strparts[i:i+N])] += 1 except: D[tuple(strparts[i:i+N])] = 1