Set Up the Apex Debugger | Force.com IDE Developer Guide

Contact Salesforce to Enable the Apex Debugger
Some services and subscriptions include the debugging feature for an extra cost. For pricing details, contact your Salesforce account executive. After Salesforce grants licenses for the Apex Debugger to your production org, match your production license counts to your sandbox.

Install or Update the Force.com IDE Plug-In for Eclipse
The Apex Debugger is part of the Force.com IDE plug-in for Eclipse. Before you set up the Debugger, install or update the Force.com IDE.

Set Up a Permission Set
Create a permission set for Apex Debugger users, and assign it to users in your org who plan to use the Debugger.

Create a Project
If you’re new to using the Force.com IDE, set up a project. If you plan to debug an existing Force.com IDE project, skip these steps.

Test Your Debugger Setup
To make sure that everything is functioning properly, create a simple Apex class, set a breakpoint in your code, and then hit the breakpoint using Execute Anonymous.

Whitelist Users and Request Types
Filter which requests are debugged by setting up whitelisting. If you don’t use whitelisting, all events in your org trigger debugging during a debugging session. Whitelist users or request types to focus only on the events that are relevant to the problem you’re debugging.