In order to program your platform using mbed compiler you will need to change your board bootloader.
Step 1. mbed developers page.
Go to the official developers page: developer.mbed.org
Step 2. Choose your platform
On top menu select “platforms” and click on your platform, like KL25Z. They have different big variety of boards, so if you haven’t bought a board you can find one that fits your needs.
On this page you can find some information about the GPIOs and some basic specifications of the board (as I have done in the hardware section).
Scroll down to the “Firmware” section and click on “Step by step firmware upgrade instructions”.
Step 3. Follow the instructions.
These instructions are well documented and easy to understand so follow them.