Sending information via USB/Serialport
I imagine Code Gorilla's given you the proper, specific answer you needed. I simply wanted to address your first (more general) ask,


  Am I able to send instruction from my computer, from another compiler, via the Arduino USB port?


... and mention some other ways I've done this before. As long the Arduino's listening to the serial port for some specific (or any) input coming in, you can use whatever you'd like to send a message to the COM port your Arduino's using. 

You absolutely are not hemmed into using solely the Arduino IDE (or it's serial monitor) for serial communication.

For instance, I wrote a Python script that communicated with an Arduino R3 (Rx/Tx) via the COM port it was assigned via the PySerial library - before coming to Python, I'd been doing this through a MS Visual C# WPF that was connected to the COM port and communicating with the Arduino (Rx/Tx) (you might need to adjust baud rates here or there, if I recall). Just to give you some ideas.