Getting a key's value in a nested dictionary
Great job so far! I have a few suggestions for your code:


Avoid writing functions with side effects such as printed output. Side effects make a function much less reusable. Instead, you may return a generator that yields the next entry. This gives the caller maximum control over what to do with the result set: print it, iterate one by one, just get the first few items without searching the entire structure, etc.
Consider adhering more strictly to Python naming conventions. For example, functions should be lower_camel_cased. Since your function returns multiple PLUs, the function name seems more accurately written as get_plus. You can remove the pass keyword in your function and pay close attention to spacing, for example in the print (data[item]) function call and your else : block.
Clean up lines like 

else :
    if item == 'PLU':
        print (data[item])


Which is logically equivalent to:

elif item == 'PLU':
    print(data[item])

Use data.items() to iterate through your dictionary's keys and values rather than only keys. This allows cleaner syntax to access values.
Make your function more general for maximum reusability. Fetching a list of values by key from a dictionary is a task that should work for any key. Why not make "PLU" a search parameter? If you want to keep the original get_plus version, write both functions and have get_plus wrap the generalized version.
isinstance may be a more accurate choice than type if you wish to allow collection subclasses of dictionary to use your function.


Here's my version for consideration:

def find_by_key(data, target):
    for key, value in data.items():
        if isinstance(value, dict):
            yield from find_by_key(value, target)
        elif key == target:
            yield value


def main():
    menu = {
      'PLU' : '234',
      'Salad': {
        'salad': {
            'ceaser':{
                'PLU': '32'
            },
            'italian':{
                'PLU': '33'
            }
        }
      },
      'Dessert': {
        'cookie': {
          'PLU': '334',
          'NAME': 'cookie ',
        }
      },
      'Appetizer': {
        'extra sauce': {
          'PLU': '61',
          'NAME': 'extra sauce',
        }
      }
    }

    for x in find_by_key(menu, "PLU"):
        print(x)


if __name__ == '__main__':
    main()