You must accept an answer (click the checkmark) on each eligible question:
- Questions must not be community wiki.
- Questions must not be closed.
- Questions must be more than 3 days old.
- Questions must have at least 1 answer.
- There must be at least four eligible questions as determined by the above rules, otherwise the statistic will not appear.
Specifically this post of yours of yours has an answer and is over 3 days old but you haven't accepted it.
As mentioned elsewhere, hovering over the "86% accept rate" text will inform you how many questions are considered eligible. For you, at this moment, it says "this user has accepted an answer for 6 of 7 eligible questions." The one above is the one you need to accept to bring it to 100%.
That being said, if it isn't answered correctly, I wouldn't worry about it. I would, however, work to gain enough rep to offer a bounty to bring better answers to your question.
See also: