I am using weebly as my website builder. I am not able to put Adsense advertisement in the vertical menu - I am only allowed to put it in the content window if I use the editor.
I assume I will have to modify the css/html and create content panel in this area (see screenshot) but unfortunately I have no idea how can I do it. Any ideas please?
Put an adsense ad under the navigation menu seems to be impossible without modify the HTML and CSS code.
To do it, you just have to export your theme (Settings > Un-published > Download the archive) and import it (Design > All themes > My Custom Themes > Import theme). An advice, to import the theme with success, read the recommandations because, otherwise, you will have error messages. After, edit the navigation menu in your new custom theme, add your adsense code, probably, in the #sidebar div and save modifications.
I hope I could help you and say me if you have any other problem.
You can insert anything on your vertical menu. I've done that 100%. I was able to insert a search button on the navigation bar and surprisingly, you can insert anything on it.
The technique is simple, just place the HTML on the Lay out area under Design Tab. Then insert the CSS on your sites header or on the main-style.css.
This is rather a long tutorial to make it clear to you. Here is my tutorial on how to insert an element to your navigation: http://www.myweeblytricks.com/2014/07/inserting-search-box-in-weebly.html
Good luck!
Another much simpler way would be add the Column element beside your content, and adjust the settings of the Adsense Element to 'Skyscraper'