The Si1145 is an I2C sensor, so there are not a lot of pins to worry about.

Power Pins

You must power the Si1145 from 3.3 VDC.

3V - this is the input to the sensor. Power with 3.3 VDC.
GND - this is the signal and power ground pin, connect to your microcontroller ground pin

I2C Pins

So you want to actually read the sensor data, these are the pins to use! This sensor uses classic I2C. 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 Flora I2C clock master pin.
SDA - this is the i2c data pin, connect to your Flora I2C data master pin.