This plugin will provide functionality you to show posts from a category into a post/page using the [ostlist] shortcode.
This shortcode accepts a category id, the order in which you want the posts to display, and the number of posts to display. You can use [ostlist] as many times as needed with different arguments. Usage: [ostlist argument1=value1 argument2=value2].
1. Download show-posts-in-lightbox.zip
2. Decompress the file contents
3. Upload the show-posts-in-lightbox folder to a WordPress plugins directory (/wp-content/plugins)
4. Activate the plugin from the Administration Dashboard
5. Open the Widgets page under the Appearance section
6. Drag the Show Posts By Category widget to the active sidebar
7. Configure the widget options to suit your needs and click Save