The HMC5883 compass uses I2C communications which require a clock SCL and a data line SDA. The AtTiny85 does not have SPI/I2C, hence instead we can use the TinyWireM library instead of the standard Wire library.
The SparkFun demo shows how to read the chip (data is signed so use int16_t).
I will update this when I have linked everything together. Most likely compass feedback will be via WS2812 LED colors.