On your own feedback profile, at the end of the page, will be a link "reply to feedback received" (this is also accessible through the Feedback Forum in the community menu).

From there. you can select from a list of recent feedback received, or put in a member name to find FB from a specific member, and click through to leave a reply.

You may find that your reply doesn't 'take', though (i.e. you'll type it out, and go through the process to actually post it as a response, but it doesn't appear on your profile - if that happens, try logging into the US site [eBay.com], to go through the process. For whatever reason, there's a much higher success rate through that site).