Consolidating a dictionary from nested dictionary
If your dictionary always has the structure {key: {key: int}}, i.e. the values of the inner dictionaries are all integers, this can be easily achieved using collections.Counter.update:

from collections import Counter

def consolidate(d):
    consolidated = Counter()
    for sub_dict in d.values():
        consolidated.update(sub_dict)
    return dict(consolidated)


if __name__ == "__main__":
    all_guests = {'Alice': {'apples': 5, 'pretzels': 12},
                  'Bob': {'ham sandwiches': 3, 'apples': 2},
                  'Carol': {'cups': 3, 'apple pies': 1}}
    print(consolidate(all_guests))
    # {'apple pies': 1, 'apples': 7, 'cups': 3, 'ham sandwiches': 3, 'pretzels': 12}


Note that Python has an official style-guide, PEP8, that recommends using lower_case both for functions and for variables. I also put this code into a function, so it is reusable, and only called it with the test case under a if __name__ == "__main__": guard to allow importing from this module without executing the test case.