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 login 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 the 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. I am lost :)
Related question: How to translate LOGIN and REGISTER?