What is the best wasy to activate an old question that I asked that wasn't answered?
The bounty process is one way of calling attention to an unanswered question: If the answer is important to someone, they would be willing to give up the reputation points the bounty costs.
Bounties can be offered on questions by others as well, not only on your own questions.
As well as Anindo's suggestion of a bounty you could also see if the question could be improved in any way. I don't know the particular question you have in mind but maybe some more background information on what you're trying to achieve would help and possibly direct links to any datasheets / documentation involved etc. Often questions get ignored if they require someone to go off searching for things.
Editing a question also brings it to the top of the active question list which many people use. Only edit if the changes are substantial and improve the question, don't do trivial edits just for the sake of bumping it.