Our pool automatically sets your worker difficulty so you don't have to set it manually. You can set it manually by following these instructions:
To set difficulty, enter the worker password as d=xxx (where xxx is the difficulty).
Recommended difficulty is your hashrate in terahashes multiplied by 1000. For example, if your worker has 12TH/s then please enter "d=12000" (without quotes) as the password.
You don't need to set worker difficulty if you are cloud mining.