To add Google ads on the forum Kunena you should install and set up a module. For this purpose, you can use ITPGoogleAdSense. This is AdSense extension for Joomla! developed specifically for integrating Google advertisements into the content.
After installation and configuration of the module, you should define its position. There are several possibilities positions in Kunena extension:
kunena_msg_1 ... kunena_msg_n, where n = number of messages in a page
New in Kunena 1.6.1
kunena_section_1 ... kunena_section_n, where n = number of section in a pagе
kunena_topic_1 ... kunena_topic_n, where n = number of topics in a page
The positions that you can use are:
kunena_bottom - if you want to put ads at the end to the forum;
kunena_section_1 - ads placed after the first section;
kunena_topic_1 - AdSense ads will be displayed after the first topic;
kunena_msg_1 - this position will put ads after each first message, when opening a new Topic.
Set position, enable the module and save the settings. Now you can check how they look.
If you have questions, you can leave a comment or contact me. I will be happy if I can be useful.