You enable the mouse int the settings. And even if you do this the keyboard controls still work. Start a race. Dont press anything in the keyboard!!!
Click with the mouse and wait for the first BOOSt sign. When it comes now you
could release the mouse and play with the keyboard and it still BOOSt well.
But You have to do this at the start of a new race unless it will not work.
I hope this will help you.