You can only install a package that has been uploaded. So open the uploaded package (right click - "Open"). You will see the package's definition. If you switch to "Statistics" you will see what will happen if you install the package.

To install the package click "Install package" and confirm the dialog. And the installation starts. This can take some time (even minutes for big packages), so please wait until the installation has finished. To see that something is happening an overlay dialog appears.

After installation you can uninstall the package, what you see as possible action "Uninstall package" in the dialog. In the "Statistics" section you see what will happen if you uninstall the package.