[cellarius]

Just as it says in the first post:

Installation

• import the product
• if you're using Template Management System, you're done. Otherwise, add $cel_photopost_canonical at the top of your headinclude template.

For the first step:
AdminCP -> Plugins & Products -> Manage Products -> Add / import product (at the bottom of the page). Then import the xml-file you downloaded.

For the second step:
AdminCP -> Styles & Templates -> Style Manager -> Choose appropriate style -> All Style options -> headinclude -> add $cel_photopost_canonical at the top.