Must be in Safari on an IOS device running IOS 10. Make sure that you have at least one card in your Wallet (you can add one by going to Settings ? Wallet & Apple Pay)..
In Safari on a Mac running macOS Sierra. You’ll also need an iPhone (not an iPad; Safari doesn’t support them yet) running iOS 10 with a card in its Wallet to be paired to your Mac via Handoff (instructions on how to do this can be found on Apple’s Support website).
You must me on the main store page an order to use apple pay.