how do i add a dog to my reservation?

Is this a confirmed booking, meaning that the sitter has accepted the reservation? If so, then you need to have the sitter involved in this.

Have you created a profile for the dog on Rover? If not, then you should do so. Then, go into your Inbox and find the the current reservation request and click on Modify. You can't ADD any pets that you don't have listed on Rover. If you are having problems, then call Rover Customer Support AT [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team].