[Creating Application Pages for SharePoint][1]

I found this (Excerpt):
 
> Note
> 
> You can only design the page in the Source view of the designer. The
> Design view of the designer is disabled for application pages.

Also found a workaround:

[Design View on SharePoint Project Visual Studio 2010][2]

> here is a trick for this little problem:
> 
> add a user control to your project. copy all the markup code over to
> this user control. design away, then copy the markup back into your
> application page to start refining your code to work as a sharepoint
> item.


  [1]: http://msdn.microsoft.com/en-us/library/ee231581%28v=vs.100%29.aspx
  [2]: http://stackoverflow.com/questions/4339149/design-view-on-sharepoint-project-visual-studio-2010