Hi quertyn,
Nice to hear that you are actually using the Lib. As you are using the Arduino Mega the change to the PIN 20 for the CLK generation might be the problem. I've checked the pin mapping sheet for the Mega (http://arduino.cc/en/Hacking/PinMapping2560) and the digital PIN 20 can not be used as "Timer/Counter OutputCompare Match Output". In my case "OC1A", which is mapped to digital PIN 11 on the Arduino Mega.
First I would recommend to use digital pin 11 as CLK (Currently the use of OC1A is hardcoded, as I didn't found a fast way to determine if a counter is attached to the choosen CLK pin and how to configure it, so that it works all the time). I will look into this problem, but in your case it should work, if you use digital pin 11 and put the Vcc function to another pin.
Secondly. Which version are you using? Currently 0.0.7rc2 is the lastest and releases before 0.0.7 have problems getting certain cards recognized. This process is much more secure in 0.0.7rc2.
Please let me know if this helps.
regards
Angus