I have an Arduino Uno Rev. 3 and I'm trying to make it so that the Arduino can send a key press / mouse action to the computer. However, these libraries are restricted to a few other versions of the board. Is there another alternative method to doing this without actually pressing any buttons/interacting with the arduino other than USB? Thanks! Link
It is actually possible, but not easy.
The main MCU on the Uno, the ATmega328, is not capable of using the USB connection at a low enough level to make this possible. The other MCU on the Uno R3, the ATmega16U2, is. You can reprogram it to act as almost any arbitrary USB device.
Since the '16U2 has no connection to digital pins 2 and up or to any of the analog pins, you would have to program it to communicate with the '328 (or an external chip) via UART, and then have another program running on the '328 that would communicate with the other pins.
Note that you will need an external programmer for this, since once the '16U2 is reprogrammed you will lose the ability to program the '328 until you restore the original programming. Using another board running ArduinoISP will work for this.