See the File->Examples->02.Digital->StateChangeDetection example. It shows how to use a global variable to keep track of the previous state of the button. That way you can tell when the button state changes.
You will end up with something like:
#include <DigiMouse.h>
const byte ButtonPin = 2;
bool ButtonWasPressed = false; // previous state of the button
void setup()
{
DigiMouse.begin();
pinMode(ButtonPin, INPUT_PULLUP);
}
void loop()
{
bool buttonIsPressed = digitalRead(ButtonPin) == LOW;
if (buttinIsPressed != ButtonWasPressed)
{
// Button state has changed
ButtonWasPressed = buttonIsPressed;
if (buttonIsPressed)
DigiMouse.setButtons(1 << 0); // Click button 0
else
DigiMouse.setButtons(0); //unclick all
}
}