Did you bother to read the Sparkfun tutorial I linked? If avrdude tells you that the stk500 is incompatible with the device, then it thinks you are using the stk500 programmer. The command line MUST inform avrdude that you are using a programmer called "usbtiny". It must also identify the target mcu.AVRDude stops with an error stating that stk500 is incompatible with the device.
Try this command to get the device signature: avrdude -c usbtiny -p t84