Start using Unlimited Sheets straight away
You'll need to click to the "install" button and the add-on will appear in your spreadsheets right away under the menu
In order to start using the new functions you need to "Execute" the Add-on in your Spreadsheet.
Click on "Execute".
The add-on works on a "Spreadsheet level", this means that if you want to use the new functions on several Spreadsheets you'll need to execute the add-on in every document.
Is just a matter of a click though!