Office not working on my elitebook?

Hello everyone is have purchased the elite book in use last few days NOw in my elitebook office not working properly. They ask me for serials key and the previous account is not loggingout out and the man who sell this elitebook say i don't know about this. I want to activate my account on this book.. Any one who knows how to solve this. I know this is not difficult but this is difficult for me because this create isssue for me. 

Thanks