The installation of the package is the same as with any other Joomla! component. Go to your site's back-end Extensions -> Manage and click on Browse. Locate the ZIP package and click on Upload and Install.
SP Mail Client is installed as any other standard Joomla! extension, from Extensions Manager by uploading the installation zip file.
To get latest releases with remote update you have to add your Download ID in extensions options, tab Remote Updates. As alternative you can download the latest release from download section of our site, and upload it from Extensions Manager.