AdSense integration is available also the the free version.
AdSense integration normally starts with Step 2 as described on that page.
You need to get and enter authorization code.

Only when you want to use your own API keys you need to click on Use own API IDs.
In this case you get fields as described in step 1.
I get redirected to the credentials page, can’t see any “step 2” being started. Is there a shortcut somewhere to start step 2?
If you are at step 1 (Client ID and Client secret) then either enter them or click on the button Use default API IDs to get to step 2.
Is there a step-guide or a video tutorial for this? I can’t figure out where to enter the ID and the secret..?
I can only direct you to the documentation page – all the steps are described there:
https://adinserter.pro/documentation/adsense-ads#integration
The process is pretty trivial as you only need to get and enter Authorization code.
Please check the instructions again and follow the steps.
In case you still have issues please attach a screenshot of your settings page showing AdSense code generator with AdSense ad units button.

What happens when you click on the AdSense ad units button?
I only get back to credentials page after saving, there is nothing indicating any step two. Thanks anyway, thought I’d try at least.
This may indicate some issue when saving settings (or javascript errors on the page).
If you still wish to use this function I can check your website (please refer to this thread).
If I click “Adsense ad units” the message “Error downloading AdSense data: 403 error” appears to the right.
Well, that explains the issue you have. Hard to say what causes it. maybe some security plugin or some setting prevents Ajax call to load the data.
Use the form above if you need help.
I don’t have premium licence at the moment, wanted to test the plugin first. Looking for solution for my clients that needs to display banners on their website. Prepared to go pro if it works, but haven’t been able to make it work yet.
I can try to help you also if you don’t have Pro. Simply refer to this thread.
do you need a login to the site?
Yes, you will have to provide that in order to check your website.