How do I add venue location automatically when user selects a venue?
EDIT: Just realized that this post is a few months old. Hopefully you already found a solution to your problem. If not, maybe this can be of use.

If I'm understanding correctly, you would like to have the address appear in the published comment, and comment is a content type. I emphasize published because this method only allows the information to appear in the published node and not within the content creation form (which is unfortunate, because I've been searching for this functionality for my own site). Assuming the above is the case, then I think the best way to do this would be with Rules. These instructions should get you close, I think, but I haven't used the functionality in quite this way before. Once installed:


Add a field in your comment content type that matches the address/location field in your venue content type
Go to admin/config/workflow/rules
Add a new rule, and under "react on event" select "After saving new content"
Add condition "content is of type", and select your comment content type
Add action "fetch entity by id", and in the data selector input/select "node:your-reference-field:nid"
Add action "set a data value", in the data selector drill down to "node:address-field-of-comment-content-type", click continue, click on "switch to data selection" in the new value area if it's not already selected, and then on that data selector field drill down to "entity-fetched:address-field-of-venue-content-type"
Save changes


This should grab the address from the referenced node before the comment is saved, and populate the comment's address field with the referenced venue's address, after the user clicks the save content button but before it is saved.  You're probably going to want to hide the address field in the comment content creation form so that users can't mess with it. It will more than likely just confuse them since they are not directly editing it. Hopefully this helps you out. Like I said before, I haven't used it in quite this way, but I have used it in a similar fashion.

You also might want to check out Entityreference Prepopulate. It allows you to fill an entity reference field automatically within the url. So, if you have a link on the venue page to create a "comment" for the venue, the you can use this module to automatically populate the entity reference field with the venue, making it so that users don't have to make the selection themselves.