The concrete5 Marketplace gives you the option to download now or, if you are logged in, you can choose to save for later.
If you choose to download now you can simply follow the steps below under "Installing a theme downloaded from concrete5.org"
If you choose to save for later, you can connect your concrete5 website to your concrete5.org account by logging in under your site's Dashboard > Add Functionality Page.
After you are logged in you should see the items that you saved in the concrete5 Marketplace. First, click the 'Download' button next to the saved item that you want to use, then after it is downloaded, click the 'Install' button and your theme should be ready to use.
In order to use your theme you will need to activate it. To do this, simply go to Pages and Themes and click 'Activate' next to your newly installed theme.
Download and unzip the folder.
Upload the entire folder to the 'packages' folder in your concrete5 directory.
Go to Dashboard > Add Functionality and click the install button next to the theme, under the 'Downloaded and Ready to Install' section.
Go to Dashboard > Pages and Themes and click the 'Install' button next to the theme.
After you have installed the theme, click the 'Activate' button next to the theme to use it on your website.