How to display elements in an element?

'{"MENTIONEES":[{"S":"0", "E":"10", "M":"xxxx"}}'

The string is valid as both Python dictionary literal and JSON.
You need to understand where this string comes from and what format it follows and treat it in the correct way.
Since the existing answer from BLUEPIXY is to treat Python as literal, I will show you how to parse as JSON.

(Python has a class called dict as the global variable dict, so overwriting it is confusing.d is used as the variable name below.)

#!/usr/bin/python3
# coding —utf-8
import json

d={'MENTION':'{'MENTION':'{'MENTION':'{'MENTION':'0','E':'10','M':'xx'}'}

# get the part of the JSON string from dictionary d
comment_json=d ['MENTION']

# Parse the JSON string and get a dictionary
comment_dict=json.loads(ment_json)

# obtain the desired value from the resulting dictionary
print(ment_dict["MENTIONONEES"][0]["M"])

dict={'MENTION':'{'MENTION':'{'MENTION':'{'S':'0','E':'10','M':'xx'}'}
subdict=eval(dict['MENTION'])
print(subdict['MENTIONONEES'][0]['M'])

DEMO