Having issues adding to project

I’m following this tutorial https://www.youtube.com/watch?v=aZ2-FQs05dI&t=1158s and I had already downloaded the dark ruins megascan before, however it doesn’t allow me to add to my project, it only lets me either create or open my previous project with this megascan. I tried migrating it, but my computer doesn’t see to be able to handle it.