Programming is a little bit special, you need the DigiSpark version of Arduino IDE (google for it!). When uploading, wait for the IDE to ask you to plug it in! If you plug it in too early programming is not possible! The bootloader simulates USB for the first few seconds after powering it, this also causes a delay in starting the program, please allow for this behavior when designing an usage. The bootloader takes some 2k of the memory so 6k remains for programming.(standard attiny85 has 8k of memory) One disadvantage: the power led is quite bright, for some applications this can be a concern (I use this controller for a WS2812 led strip project, the red is showing quite visible when the strip is off)
You can download the digispark IDE here:
https://digistump.com/wiki/digispark/tutorials/connecting