sf::RenderWindow window(sf::VideoMode(320, 240), "Font Test");
...
text.setPosition(250, 250);
It looks like you are trying to draw your text outside of the window. Your window height is set to 240 pixels, but you are drawing text with a Y position of 250.
Also, make sure you check the return value of font.loadFromFile("ditr.ttf") to make sure it doesn't fail.