The SI1145 is an I2C sensor, so there are not a lot of pins to worry about.
You can power the SI1145 from 3 to 5VDC, there's an onboard level shifter to safely bring the voltage down to 3.3V.
Vin - this is the input to the voltage regulator. Power with 3-5VDC. Has reverse-polarity protection.
GND - this is the signal and power ground pin, connect to your microcontroller ground pin
3vo - this is the output from the onboard regulator. If you need a clean 3.3VDC, you can draw up to 100mA from this pin.
These are pins that you don't need to connect to but can if you want to use the more advanced features of the chip.
INT - this it the interrupt output from the chip, with a 10K pullup to 3.3V. You can check this pin to see when the next set of conversions are done. Our library doesn't use it
LED - if you'd like to try using the proximity capabilities of the SI1145, you can tie an IR LED to this pin. Any IR LED will do. It's a open collector pin, so connect the cathode (-) pin to this pin, and the anode (+) pin of the LED to 3 to 5VDC. It can sink up to 300mA in a spike so be aware of that if using this pin
So you want to actually read the sensor data, these are the pins to use! This sensor uses classic I2C so any I2C-capable microcontroller can use it. These pins are level shifted so you can use 3 or 5V logic safely. There's 10K pullups on both pins. You can share the i2c pins with many other sensors as long as they do not use the same address - the SI1145 uses 7-bit address 0x60
SCL - this is the i2c clock pin, connect to your microcontroller I2C clock master pin.
SDA - this is the i2c data pin, connect to your microcontroller I2C data master pin.