To add your activity or ideas, click the purple + sign in the lower right-hand corner of the screen. If you haven't used Padlet before, I have included a quick screencast above that goes over the essentials.

And to add you comments to another person's Padlet, click the "Add comment" + sign at the bottom of there post, and you will then have the option to add feedback using text, video, audio, and several other options.