SharePoint Designer "failed to load this workflow" or Windows Workflow Foundation, part of .Net Framework 3.0, must be installed to use this feature.

To resolve this issue please update your designer with the hotfixes in sequence. Make sure the designer is NOT open.

http://support2.microsoft.com/hotfix/KBHotfix.aspx?kbnum=2837667&kbln=en-us

http://support2.microsoft.com/hotfix/KBHotfix.aspx?kbnum=2727100&kbln=en-US

Once installation is complete open the site once again. A restart may not be necessary. If you still get the same issue then clear your designer cache by running this 

rmdir "%LOCALAPPDATA%\Microsoft\WebsiteCache\" /s /q

rmdir "%APPDATA%\Microsoft\SharePoint Designer\ProxyAssemblyCache\" /s /q

Then Restart you PC.