This should work. With index and rnd as variable.
Every click on the button there should be another key in the text-field.
+ Button: On clicked
-> System: Set rnd to int(random(0,Dictionary.KeyCount))
-> System: Set index to -1
----+ Dictionary: For each key
-----> System: Add 1 to index
--------+ System: index = rnd
---------> Text: Set text to Dictionary.CurrentKey