To link your minecraft username you must first make sure you have a mojang account for your minecraft account. This will tie your account to an email address so you log into your minecraft account via your email address and minecraft password. When this is done you need to ensure that your email address on the board is the same email address that is linked to your minecraft account. To do this visit your User Control Panel and navigate to the profile tab subsection edit account settings. You will also be able to change the email address on the board. When this is done you will be able to fill in the information below supplying your forum password and minecraft password and successfully link your minecraft account.