In order to use floating point numbers in printf/stdio functions, in Atmel Studio 6.1, press Alt-F7 to open the project properties.
Under the toolchain tab, click on AVR/GNU Linker > General and select the box labeled Use vprintf library.
Next, click on AVR/GNU Linker > Libraries and add the library titled printf_flt by clicking on the green Add Item button.
Th-Th-Th-That's all, folks!