Datasheet

And type in adafruit tsl2591 to locate the library. Click Install

We also have a great tutorial on Arduino library installation at:

http://learn.adafruit.com/adafruit-all-about-arduino-libraries-install-use

Install Adafruit_Sensor

The TSL2591 library uses the Adafruit_Sensor support backend so that readings can be normalized between sensors.

Search the library manager for Adafruit Unified Sensor and install that too (you may have to scroll a bit)

Load Demo

Open up File->Examples->Adafruit_TSL2591->tsl2591 and upload to your Arduino wired up to the sensor