How to start Start Profile Synchronization in SharePoint 2010. Before we can start the profile synchronization, make sure your SharePoint 2010 site is ready for profile synchronization. For more information, please see : http://virtualizesharepoint.com/2011/12/09/how-to-setup-user-profile-service-in-sharepoint-2010/
1. Central Administration| Application Management | Manage service applications | User Profile Application
2. Select Incremental Synchronization to start an incremental synchronization now. Only data that has changed in connected sources and User Profile will be synchronized.
Not recommended: In most case, Incremental sync should be sufficient. Selecting Full Synchronization is time and compute intensive and is not recommended unless absolutely required to reset data store in User Profile.
3. After fully synchronization, SharePoint was able to get all the users in Active Directory