Firstly make sure that Scroll control is selected for use with the iD button. This can be done by clicking on the iD icon found in the master section of the iD app and selecting ScrollControl.
To activate Scroll Control simply press the iD button
Now if you hover your mouse over something that you can scroll with your mouse wheel, when you rotate the encoder, it will scroll it. This could be a parameter on a plugin, a fader in your DAW, or even a webpage or iTunes library (ANYTHING that is scrollable).
Some computers use velocity sensitive scrolling. This means that depending on how fast you turn the encoder, you will scroll by a different amount. So turning it very slowly can allow fine adjustment of parameters, or fast movement can make big jumps. This can be a little strange at first, but you soon get used to it and you can use it to get some really creative results.
It is worth noting that how well ScrollControl works, is dependant on how the plugin/DAW/software handles mouse wheels, so you may find that certain software works better than others. One thing you can do, is if you find that ScrollControl is too sensitive or not sensitive enough, changing your scroll speed in your computer settings can help.
Here is a video giving you some ideas as to how you can use ScrollControl: