An extension or theme in the marketplace will be installable from any Pagekit admin interface.
Before you can upload a package for the first time, you have to create a account at pagekit.com.
Log in on pagekit.com and navigate to the Developer section. Hit the Submit Package button and upload your package.
Note Be careful to have your files on the root level of the zip file or the file will not be considered a valid package.
Hit Submit and you will be presented with a settings screen for your new package. Notice how the Status of the package is set to Unpublished by default.
When you have finished a new version of your extension and try to upload it to the marketplace, make sure to increase the version number in the composer.json file.
Version numbers must comply to a certain naming scheme like explained in the PHP docs. A simplified (and incomplete) regex for a valid version number is (with x being a number):
x.x.x(-(pre|beta|RC|alpha))?x?
This is what correctly applied version numbers could look like: 0.0.1, 1.0.0, 0.1.2-alpha, 0.1.2-alpha-2, 0.1.2-RC2.