It happens that when I connect my arduino to my PC, it works, but the problem is that the RX (led) doesn't work, so it just don't recieve the progamation. I'm using it on S4A (Scratch for Arduino).
If the RX LED doesn't blink there is no data arriving from the PC. How do you tell Scratch for Ardwino which serial port to use? You might want to try installing the Arduino IDE and uploading the basic Blink sketch to see if the problem is with your PC or with S4A.
I have tried that, but the problem seems to be the arduino instead of the pc or S4A. I'm asking if the RX problem has some solution? or do I have to buy another one?
If your Arduino UNO never worked you should contact the seller to request a replacement. If it worked before but doesn't now the USB-to-Serial circuit may be damaged. If the chip had failed your board would not be recognized as a USB Serial Port. Perhaps you can find someone familiar with electronics to examine the circuit.