wher's the USI on the ATTINY?
is it two wires?
where to I connect it to on the Arduino?
but one thing
the ATtiny85 doesn't have enough I/O pins to do what you want.
Wouldn't a shift register be useful here?
You could use a serial connection between Arduino and the Tiny:
But you don't need MISO because you are only talking one way.
It only have 5 pins which means you could only have 3 LEDs if you used 2 pins for communication.
however this requires either a High voltage programmer or a bootloader (The latter of which takes up 576bytes of flash).
You can do it with an ATtiny, but you have to charlieplex the LED`s.http://uzimonkey.blogspot.co.uk/2009/01/charlieplexing-on-arduino.htmlhttp://www.instructables.com/id/Creating-a-charlieplexed-LED-grid-to-run-on-ATTiny/
mixania, just use a shift register and save yourself a lot of pain
add 8 16bit I2C GPIO expanders, and use charlieplexingthere you go, direct control for 16256 LEDs, using only two pins of your Arduinoenough for a 25 x 25 x 25 LED cube