I got your slider as part of a theme from Theme Monster and when I replaced some of the slides with new images, there appears to be no way to make them slide in. I go to choose the animation, slide is not an option. Lightspeedin is the closest option. I really like the smooth slide without the goofy distortions.
Make sure there is installed latest version of slider. At this time it is 2.0.0. Do you have live example of slider and animation you ask about? Could you specify parameters you applied for layer/slider?