The solution to use the function keys is pressing the key fn. To press F5, you need to press fn+F5. It works but it's quite inconvenient. A better solution is to use FunctionFlip. It's a free program that let you use some function keys without the need to press the fn key. In this case F5 would be F5, and if you want to dim your screen you would need to use fn+F5. You can find it here:

It's very easy to use and works well. Remember to add it to the list of programs that start when you log in.

Javier