Navigate to the Webwinkelkeur Connect on Magento Connect. See: http://www.magentocommerce.com/magento-connect/colorbox-a-lightbox-image-gallery.html
Check if the extension is compatible with your store's release version and platform. Click on "Install Now". Check the box to agree to the extension license agreement. Once you have selected the channel and agreed to the licensing agreement, click "Get Extension Key".
In a new window, login to your magento Back-end and locate the Magento Connect manager at System > Magento Connect > Magento Connect Manager.
Paste the Extension Connect code into the installation field and select "install". When the extension has finished installing, you may hit the "return to admin" button to go back to the Magento Back-end. You can verify the installation by making sure that its functions are available in your Magento Back-end.