@ mark nguyen,
So Far we know you didn’t download the Easy Extend Theme. If you want the Slides for CORPORATE Theme you can do it in two ways.
You can show the Slides either from the images of WP-Admin > Appearance > CORPORATE Options > Slides or can show from Posts/Pages.
You can upload images in the WP-Admin > Appearance > CORPORATE Options > Slides and Save Options
or FROM POSTS/PAGES
You can upload only one Featured Image in a post/page. You can use the Featured Images for all posts which you want to show in the Slider. Suppose You have the following Posts/Pages
1. A nice article 01
2. A nice article 02
3. A nice article 03
4. A nice article 04
5. A nice article 04
You can add only one Featured Image for First Post. But you can add Featured Images for all the 5 posts separately. Then if you check the box Sow in Slides for 1, 3, 4, 5 posts those will be shown in the Slide. 02 will not be displayed in the Slide.