Make sure you have the Plugin zip file that has been provided to you with the installation files.
1 - Login
Login to 1gateway, and make sure you are in "Advanced mode". If this is the first time you log in, the default user and password are both 'admin' (without the quotes).
2 - System Maintenance
Click on the menu icon and choose the option "System maintenance"
3 - Upload file
In the System maintenance view you can either drag and drop the downloaded zip file or select it by clicking the "choose file" button.