How to associate multimedia keys to Foobar2000?

For Play/Pause:

File -> Preferences -> General -> Keyboard Shortcuts

Click Add New and setup the binding you want. Make sure to select Global Hotkey.

As for launching from a keystroke, when you edit the properties of a shortcut you can assign a Shortcut key.

Multimedia keyboards + foobar2000: A complete guide detailed everything that I wanted to do with the keyboard.