Pretty soon after using SharePoint Designer, you’ll notice some quirks. One quirk is that sometimes, when you try to use the "Insert" menu option, most of the options are grayed out. See the image below:
You will use the Insert option when trying to add such items as a Web Part Zone to your page layout, for example.
Luckily, the solution to this problem is simple: Save the layout before trying to insert the module. SharePoint Designer grays out these options if there are pending changes. Don’t ask why, just save it out and you are on your way.