working on project 3 in project book

the project book has the following line in the code

void setup(){
Serial.begin(9600); //open a serial port

i am working on a mac and am using usbmodem621 How do I declare it instead of "Serial.begin(9600);"

You don't need to. That code is running on your Arduino board, which doesn't know or care about usbmodem621. It's the computer that cares about that. So you only need to make sure you have the right port selected in the Arduino IDE or Arduino Web Editor and you're fine. Serial.begin(9600); works universally. The 9600 is the baud rate.