I want to say "You need activate added user." but I don't know why I say it. I don't know any rule for which I can say added user.
Past Participles are used as attributive: adjectives placed in front of a noun.
The stolen baby was found by the police unharmed.
Dean's broken arm was set in plaster by the doctor at the hospital.
The required steps are clearly listed in the email I sent you.
In your case, you could say "You need to activate the added user." if you are talking of a user account previously added that needs to be activated.
Most past participles can be used as adjectives. An added user is a user that is added or that has been added. So you can say "you need to activate the added user”. Note that you need to use the article the since you are refering to a specific user that has just been mentioned.
However I find this formulation awkward, although I can't articulate why. I prefer "the newly added user” (or more wordy alternatives such as "the user that you just added” or "the user [who was] added in step 3”).