If your project is a Maven project, it should have a file called pom.xml in its root directory. This is the Maven project descriptor.
In the POM there should be a section <dependencies> to which you can add a <dependency> element for the library that you want to add to your project. The second answer in the StackOverflow page that you linked to has an example of such a dependency.
Before you add any dependencies at all, please tell us why you need Apache POI. If you're not working with Microsoft Office files, then using Apache POI might be overkill if all you intend to do is read a CSV file.