The sensors use 2.5V, so also the I2C signals of the sensors are 2.5V.
The level shifter protects the sensors from the 5V signals from the Arduino, but also raises the 2.5V signal from the sensors to the 5V signal of the Arduino. It's in both directions.
To be able to do that, the level shifter needs the high voltage (5V) and the low voltage (2.5V). That's why I wrote that the low voltage must be connected to the 2.5V on the sensor board somewhere.
If you take a look at the websites of Sparkfun and Adafruit, you will find more suitable sensors for the Arduino.