Taking as an example tonight's London Photowalk event, we need to first get the id of the activity. This can be done by grabbing the public posts by the Photowalk account:
The ID "111455345092279041936" there is the ID of the Photowalk account. From that list we just need to find our event, and get the ID. You can generally see the Events posts by the provider field in the JSON of the object attached to the post.
We can then request the list of comments for that id using the plus.comments.list functionality.
Take a look at the results over on the API explorer.