Using ATiny with I2C

I’m trying to use an I2C OLED display with an ATtiny85 chip. First I needed to find a library that makes it possible to run I2C on the ATtiny, and I found this: http://playground.arduino.cc/Code/USIi2c. However, the setup of the library and the description confuses me. When I download it, there’s both a library (h and cpp files) named TinyWire and a set of cpp/h files named USI_TWI_Master. The latter is not mentioned anywhere in the description of the library.

What is it I don’t get? Do I need both or one of them?

Also, related: when I add the library Liquidcrystal I2C (ATtiny version), there’s an #ifdef statement saying:

#if defined(AVR_ATtiny85) || (AVR_ATtiny2313)
#include “TinyWireM.h” // include this if ATtiny85 or ATtiny2313
#else
#include <Wire.h> // original lib include
#endif

How and where do I define the board type? Just selecting it as board doesn’t seem to do the trick.