Not able to find _LAYOUTS folder in SharePoint Designer
The _layouts folder contains Application pages, and this type of pages can't be customized by the SharePoint Designer. If you want to customize them, you will need to have physical access to your SharePoint server since they are located in the folder C:\Program Files\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\LAYOUTS
Be aware that the files in this folder are used by ALL SharePoint sites and lists, so one change here can have a huge impact! Additionally modifying OOB Application pages is not recommended
