грамматика

[Али]

не помню куда вставляют по правилу successfully
since... has been successfully working or
since... has successfully been working

[THE RED]

Обычно/чаще между вспомогательным глаголом и изменяемой частью сложного времени стоят немногосложные наречия, напр., just, yet, already и подобные.

[CH3CH2OH]

I remember someone telling you to use google. Although google's not a rule, and search results should be used carefully, but in this case they're quite helpful

http://www.google.ru/search?sourceid=navclient-menuext&ie=UTF-8&oe=UTF-8&q="has+been+successfully+working"
http://www.google.ru/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q="has+successfully+been+working