Paul, I think you're having the same struggle I had for a while in the new community - When you are looking at this thread in your inbox/activity, it allows you to reply but doesn't give you full functionality. You have to click on the link at the top of the thread and go to the event page, and then it should let you add attachments.
EDIT: It seems like you have to actually edit the event itself (as the owner of the event, you should see an edit button on the right side) and attach the file. Doesn't seem like it can be done via a reply.
That said, THANK YOU for linking to your PowerPoint here, so I could quickly download it!