This extension requires VQMOD installed on your opencart.
Extract the zip package xml-sitemap-pro-15xx-x.x.x.zip
You will find the folder upload
Copy / Drag the files & folders inside the upload folder to your web root directory such that it merges with the existing admin, catalog, vqmod folder. (No core files overwritten)
Go to Admin > System > Users > User Groups, and give appropriate permission to extension/hb_sitemap