You can use pullup pins with buttons, the resistors are built into the AVR chip. For quick work just plug jumpers into the pin holes and to press a button, touch it to the USB jack housing which is GND.
The pin is set to INPUT and HIGH. The HIGH comes through 20k-50k. When it is not grounded it reads HIGH, when it is grounded it reads LOW. That's the opposite of 5V-->resistor-->button-->pin.