10 years ago.

Your target should also have a helloworld program which should work on its information page (https://mbed.org/platforms/FRDM-K64F/, right side). Still that one needs an edit: I know mbed staff can make programs always include the latest versions of libs, but that one doesn't have that feature yet. So a very old version of the mbed lib is included, which does not support the K64F yet.

Now just importing helloworld program from the link above will work, but you are going to run more often into this, since you have a relative new board: When example programs are made the version of the mbed lib used doesn't support it yet. The solution generally: Click on the mbed lib in your program in the compiler, now at the right side there is an update button. Press it, and you get the latest version. This can sometimes also be required for other libs (for example ethernet for ethernet examples).