First, make sure that you have shared the presentation for public access, so it says Anyone on the internet with this link can edit:
Only add the Slides itself
If you prepared a Presentation before and you just want to go over the Slides, click on Add in the top left corner and insert the link under the Google Slides section. This can be done in the Playbook itself and during a meeting.
Edit the File during the Meeting
If you want to interact with your guest and edit Google Slides, Sheets or Docs please add the link in the Website section. The Link can be included in the Playbook or uploaded during the meeting, this allows you to make changes to the file during a Session:
If you want to give your guest the right to edit the file together with you, please make sure to Share control using the following button - the arrows should turn Blue.