If you’ve installed both SharePoint Designer 2010 and SharePoint Designer 2007, you might run into this problem:
You’re on a SharePoint 2010 page and want to edit it in SharePoint Designer 2010. You switch to the Page menu on the ribbon, expand the Edit Page button and click the Edit in SharePoint Designer menu item.
You’d expect SharePoint Designer 2010 to start up and open the current page for you. Instead, SharePoint Designer 2007 starts up. Which is no good to you, and quite annoying!
There can be multiple ways to fix this, but by far the easiest is to go to your Windows Control Panel, open the Programs and Features list, locate Microsoft SharePoint Designer 2010 in this list and click the Change button. This opens a Setup window with several choices. Close all MS Office programs and choose the Repair option. Wait for a few minutes and after it’s finished, reboot your machine.
The correct settings have now been set for SharePoint Designer 2010 and you should be able to start it directly from the SharePoint pages.