The Matrix consists of 64 RGB LEDs and each LED needs about 60mA (3 * 20mA) at full brightness. Therefore the maximum current is 64 * 0.06A = 3.8A.
The ATtiny85 is connected via SDA+SCL (I2C) to the Raspberry Pi.
For direct control of the WS2812 LEDs from the Raspberry Pi have a look at the examples from: 626Pilot and jgarff. The jumper next to the reset switch has to be closed to position 18. Then the data input of the first WS2812 LED is connected to GPIO18 of the Raspberry Pi.
The EEPROM on the HAT can be updated as follows. Before the update the WP jumper next to the EEPROM has to be closed.
Install EEPROM utils:
$ git clone --depth=1 https://github.com/raspberrypi/hats
$ cd hats/eepromutils
$ make
$ chmod +x eepflash.sh
Generate EEPROM data:
$ wget https://github.com/watterott/RPi-WS2812-HAT/raw/master/software/rpi-ws2812-hat.txt
$ ./eepmake rpi-ws2812-hat.txt rpi-ws2812-hat.eep
Update/Flash EEPROM:
$ sudo ./eepflash.sh -w -t=24c32 -f=rpi-ws2812-hat.eep
Note: For a Device Tree Kernel add to /boot/config.txt the following line before flashing: dtparam=i2c0=on (before any dtoverlay statement).