The classes for the OOXML file formats (such as XSSF for .xlsx) are in a different Jar file. You need to include the poi-ooxml jar in your project, along with the dependencies for it
You can get a list of all the components and their dependencies on the POI website here.
What you probably want to do is download the 3.11 binary package, grab the poi-ooxml
jar from it, and the dependencies in the ooxml-lib
directory. Import these into your project and you'll be sorted.
Alternately, if you use Maven, you can see here for the list of the artificats you'll want to depend on, but it'd want to be something like:
<dependency>
<groupId>org.apache.poi</groupId>
<artifactId>poi-ooxml</artifactId>
<version>3.11</version>
</dependency>
The poi-ooxml maven dependency will pull in the main POI jar and the dependencies for you automatically. If you want to work with the non-spreadsheet formats, you'd also want to depend on the poi-scratchpad
artifact too, as detailed on the POI components page