You can tell Adsense about a keyword through the content of your page on which that adsense unit is place. Use the keyword on that page appropriately.
You should write your content with that keyword blended in the post and you should be fine telling adsense about the importance of that keyword.
In the page where you want to place adsense code : 1. set the metatag correctly yo reflect your keyword (keyword, description, title) 2. The content - place relevant content on your page 3. Backlinks - When you place your links on other websites, use your keyword as anchor text to point back to your page