In order to use your Merchant Shop, you'll have to be a Merchant. Your friend can check to see if he is a merchant on the sidebar under his avatar next to "Class:". You can either be a Rancher and have a Rancher Shop (which stocks Creatu) or a Merchant. If your friend is currently a Rancher and wants to switch, that can be done a limited number of times by clicking the Preferences button on the sidebar and clicking "Character Class".

If your friend is a Merchant and has not yet created a Merchant Shop, I believe that can be done by hovering over the Shops tab near the top of the page and clicking "Merchant Shop". And finally, items can be added to your Merchant Shop by clicking the Stock Shop link from your Merchant Shop and clicking the User Shop bubble next to the items you want to stock.

I hope this helps your friend! :)