The above is a frequent error and question that gets asked by many in the Kunena community. I hope the following short tutorial will help clear things up for those who are trying to install a new custom Kunena template.
Step 1
In the backend of your Joomla site (joomla administrator) locate the Kunena template manager.
Components > Kunena Forum > Template Manager.
Step 2
In the Kunena Template Manager locate the add button in the top right hand corner and select it.
Step 3
Browse for the custom Kunena template you would like to upload.
Step 4
Select the compressed file and click open.
Step 5
Click on the upload and install button.
After installing you should now see your custom template in the Template Manager selection.
Step 6
Last but not least don't forget to select your new template as the default theme.