Your post is worded slightly ambiguously - I'm not 100% sure if you're trying to put dictionary data in a JSON file or load a JSON file into a dictionary. I'm guessing it's the latter?
There's two parts to this: getting the contents of the file, and putting them into the dictionary.
To get the contents of achievements.json, you can use the AJAX plugin > Request achievements.json (tag "loadAchievements")
AJAX is asynchronous, meaning this request will take a small but indeterminate amount of time to complete. That means we have to test to find out when the data has been loaded and is ready to use.
AJAX > On "loadAchievements" completed: Dictionary > Load from JSON string AJAX.LastData
Note the Dictionary accepts JSON in the following format: