Can't enter magic portal at end of ch 1

I'm playing the iPad version, and after getting the nexus stone and installing it, I can't find how to enter the portal. Where do I touch to activate it and finish the chapter?

I have the same problem. At the end of chapter 7 I can't place the stone on the portal. I'm also using an iPad. Help

I have the same problem at end of chapter one. I accidentally went to the lobby and the portal stopped glowing upon my return. Do I need to delete my game and start all over ? Also using iPad (4th gen)