You can install Sendit in two ways:
Classic wp-admin installation
From your wp-admin plugins sections go to add one and just search for “sendit” and let WordPress install for you. Probably this is the most common and fast way to get it. To install Sendit Pro you need at least Sendit 2.0.5
Download from here or from WordPress plugins repository
Get it from my italian blog or get it directly from the official wordpress newsletter plugin page.
Upload into your wp-content/plugins folder and activate it.
Buy Sendit Pro Scheduler and extend Sendit to send more newsletter without outbound limit
Buy Sendit pro Scheduler and install as a normal plugin (copy in to your wp-content/plugins/ folder) so you will have Sendit and Sendit-scheduler enabled to work together.
Activate it from wp-admin!