The computer terms login/logout are commonly translated as вход/выход or войти/выйти. I am struggling, however, with the expressions:
You are not logged in
Please log in first
I could translate the former as вы не вошли, but вошли куда (where)? The same with the latter: Пожалуйста, сначала войдите — again, enter what?
In English, the verb to log in implies a computer environment. In Russian, the corresponding word входить has no such connotation. I could say войти на сайт, but that would be wrong again, since the user is already in my site, he should just log in.
Related question: How to translate LOGIN and REGISTER?