You have to register the function that handles the callback function to be executed when a button is mouse pressed during your display() loop. This has to be done before running the main loop, when you are initialising everything that GLUT requires.
Then you need to define the callback function itself, for example, something like this.
void MouseButton(int button, int state, int x, int y)
// Respond to mouse button presses.
// If button1 pressed, mark this state
if (button == GLUT_LEFT_BUTTON)
g_bButton1Down = (state == GLUT_DOWN) ? TRUE : FALSE;
g_bButton1Down is the global variable that you have to check to see if the mouse button is being pressed or not inside your main display() GLUT loop.
Take a loop at the glutMouseFunc specifications.