Mobile sites are optimized to fit onto a smaller screen, so it may be hard to see where any information is.
If you scroll to the top of the page, you will see 3 horizontal lines on the left-hand side. When you tap on those, the list of pages on the website will drop down, and you will be able to select which one you want to view.
If the couple has a registry on their website, just tap on the word "Registry" and it will open the page up. Once there, you can tap on the item or items of your choice and add to your cart!