The Slider called the Featured Slider is built into the Mystique WordPress Theme you just need to configure it.
Go to the Appearance Panel > click Mystique Settings > Click on the Content Menu > Go to Featured Posts option > select / enable the options you want (be sure to set the "don't show posts older than X days" correctly).
You now need to make the posts you want to be featured, featured. Click on your Posts Panel > click on the star for all the posts you want to be featured under the featured column.
To add featured images to your posts that you have featured > Go into each post that you want featured in the slider > Set featured image