Hello ,
The root folder is the folder you download , for example for the SDK_2.0_FRDM-K64F,
when you download it , it is a compressed files "SDK_2.0_FRDM-K64F.zip", please unzip it , when you create project,
the path is the folder "SDK_2.0_FRDM-K64F" (you can put this folder to anywhere ):

Hope it helps
Alice