Add-ons may be supplied in the compressed ZIP format and need to be extracted (decompressed) before they can be imported into your Affinity app.
To extract assets from a ZIP archive, double-click the .zip file.
If the archive contains one file, the file is extracted to the same folder as the archive. If you're unsure of the file's name, (in Finder) select View>Sort By>Date Added to help identify it.
If the archive contains multiple files, they are extracted to a subfolder alongside and named after the archive.
To extract assets from a ZIP archive, right-click the .zip file and select Extract All, and then follow the on-screen instructions to extract the archive's contents to your desired location.