Under Wireless -> Splash page, You either have or would create a Custom Theme.
From there you edit that theme. Choose the HTML file that matches what you are working with. (likely Continue.html)
Edit the HTML Code to move the "button" where you want it in the page.
** <a class="button" href="$MERAKI:AUTH_AND_CONTINUE_URL$" title="Continue to the Internet">$MERAKI:CONTINUE_TO_THE_INTERNET$</a>