The enhanced page layout editor is a feature-rich tool that allows you to customize your organization’s page layouts for detail and edit pages in Advologix and the Client Portal. The enhanced page layout editor provides functionality in an easy-to-use interface.
The enhanced page layout editor consists of two parts: a palette on the upper portion of the screen and the page layout on the lower portion of the screen.
The palette contains the user interface elements, such as fields, buttons, links, related lists, and any additional elements that are available for you to add to the page layout.
Navigate to Setup
Click on Your Name
Select Setup
To Edit Custom Objects page layouts, after you click on Object link, you will need to select the object to your wish
Click the name of the object in the list.
Click on Page Layouts.
Click on Edit link next to Matter (Custom Object) layout to open.
For further information, See Force.com's: Customizing page Layout with the Enhanced page Layout editor