The posting process is a little different with Instagram because they don't allow outside apps to auto post to them. After you press share to Instagram, you first have to wait for your graphic to finish uploading into your Ripl feed, and then you will see a prompt within Ripl that asks you to "Launch Instagram."
When you click "launch Instagram," you'll be taken to Instagram with your camera roll open. Be sure that you're logged into the correct Instagram account prior to posting. Your video or photo will be loaded. When you get to the share screen, your caption has already been copied to the clipboard. Just tap and hold to paste in the caption box and complete your post!