Hi,
in your posts page select the post you want to use a featured image with and then on the right hand side below your categories window you will see a link called set featured image. Click this link and select an image from your library or upload one from your pc.
You may choose to edit the image if you want.
then go to your main post page and make a note of the ID number for the post you have just uploaded a featured image in.
Then in your backend go to appearance > featured slider > slider options
in the featured col# 1 box enter the ID number of the post and click save
your featured post should now be working