First download the chosen package, either directly from the marketplace website or from the author’s pages by clicking the Download button.
Then check the downloaded package:
If it is an .exe file, execute it and follow the instructions provided in the installer.
If it is a .zip package containing only folders and .export files, then import the whole .zip package into Kentico using the Sites -> Import site or objects button.
If it is a .zip package containing another .zip package (and other files, such as instructions, documentation, libraries, etc.), then unpack the main package. After that, import the sub-package into Kentico CMS using the Sites -> Import site or objects button.