The frosting can be whatever flavor you enjoy the most.
I chose cream cheese because of the neat color contrast between the cake and pop rocks (and because it's tasty). I will not judge you for choosing chocolate, though.
As for the pop rocks, the Orange flavor is a special edition flavor. If you cannot find them around town, you can buy them on Amazon.
Note: If you don't want to buy pop rocks from Amazon, you can attempt making your own. Though, without some serious machinery, your pop rocks will be more like fizzy rocks. Recipe here:
http://bit.ly/1J1qsiy If you want to make frosting from scratch, I recommend following this delectable recipe:
http://fd.cm/1J1qebf