Sir
the problem is in the new version of software if you use the AVR studio 4 you will came to know that if you select the uC and include avr/io.h some necessary header is automatically included in the project, which is can be seen in the project file...
But in AVR studio 6... that file is not showing in it. it work on backhand. Try studio 4. also that is also a good s/w...