Have you tried using the presets? If you have purchased the software, take a minute to register it, and you will have unlimited access to the presets library here on the site:http://fxhome.com/presets/
A quick search for "fog" should give you some useful options.
If you are working with the demo, you can't access and downlo0ad new presets, but there might be something included with the demo that will prove useful to you. Searching through the presets in the Particles list is your best option. If you need further assistance, please provide additional details about what you need help with, specifically, and we'll do what we can to get you up and running.