Are you having issues opening a page layout in SPD 2010 for editing? Is everything highlighted in yellow? John White has a good explanation of what your problem probably is. You need to edit the file in Advanced Mode. What’s Advanced Mode? It’s opening the file so you can really edit it instead of opening it in a mode where you can’t edit it…even though you clicked on Edit File. Perhaps it should be called "Really Edit the FIle" (That was supposed to be tongue-in-cheek humor).
What’s interesting is sometimes you’ll be asked if you want to open the file in Advanced Mode and other times the file will open in "read only" (yellow) mode. Call me old fashioned, but when I click Edit File, I intend to edit the file.
Right-click the file and select "Edit File in Advanced Mode."
Or, with the File Information screen displayed, select the Edit File drop-down and click on "Edit File in Advanced Mode."
Either way, you’ll be able to edit the file, as you probably intended to do in the first place.
Can’t edit file in SharePoint Designer 2010
Can’t edit page layout in SharePoint Designer 2010
Page is locked for editing in SharePoint Designer 2010
After copying and pasting a page layout it can’t be edited
Edit File in Normal Mode vs. Edit File in Advanced Mode