From the left-hand menu, select 'Plugins' then 'Add New'.
Select 'Upload Plugin'.
Choose file you have downloaded.
Activate the plugin.
Go to 'GSpeech' menu from the left-hand menus.
Place the 'Widget ID' you got when created a website. See how to create a website. To get 'Widget ID' of existing website, go to your dashboard, click on the website, under 'Settings' menu you will see the 'Widget ID'.