Try setting up your own weather situation with temps and dewpoints real close to freezing. Then, as you ascend, you should get into colder temps and you should get some icing effect. It's not modeled as something you can see in the default aircraft, but I've seen some that do have icing modeled.

There's a discussion on icing and the building of a gauge that detects icing on another forum I hang out on:
http://bgaforums.proboards.com/index.cgi?board=fs9&action=display&thread=201