Upload the addon to the directory where coranto.cgi resides. This is usually done with an ftp program.
Once the file is on your server you go to the Coranto Administration page and select Addon Manager.
Each available addon will be listed along with the status of the addon (enabled or disabled).
Toggle addons on and off by clicking the enable/disable link.
Some addons add a menu item to the Admin page like XMultiSubmit and Snipper. Select them from the admin page by clicking on the addon name and configure these addons from their own settings page.
Other addons place settings on Coranto's main settings page. Multipart does this.
In addition some addons provide additional settings on profile pages or in other places in Coranto. Make sure that you read the documentation for each addon.