Do you see the Ecwid plugin listed in 'Plugins' section? If so, just enable it by clicking the 'Activate' link and then set your store ID on the 'Settings -> Ecwid shopping cart' page. If you do not see the Ecwid plugin in that list, it means the plugin hasn't been installed yet. Please find the further instruction below.
Since you already downloaded the plugin zip archive, you can simply upload it to your WP installation via your admin panel. How to do it:
- Go to WP admin panel → Plugins → Add new
- Click 'Upload' link
- In the appeared dialog window, choose saved zip file on your computer
- Click 'Install'
Then you will need to activate the plugin on the Plugins page (click 'Activate' link) and configure it on the 'Settings -> Ecwid shopping cart' page (at least, you will need to set your store ID there).
By the way, in order to install the Ecwid plugin it is also possible to use the WP automatic plugin installation as described here:
Hope it helps.