Atlassian’s document describes how to install apps, including Excel-like Tables for Confluence:
Manage your apps (Confluence Cloud)
See Find and add new apps section for the details.
You can also add it from Atlassian Marketplace. Go to marketplace.atlassian.com and search Excel-like Tables for Confluence. You will find Try it free button at the upper-right corner.
Atlassian Marketplace: https://marketplace.atlassian.com/apps/1225075/excel-like-tables-for-confluence?hosting=cloud&tab=overview
Product page: https://www.ricksoft-inc.com/excel-like-tables-for-confluence/