To use a TGC, use select File / Insert Clip File and then find the TGC file associated with what you've downloaded.
In the default TGD, you'll find the default atmosphere. If the TGC you inserted does not have an atmosphere attached to it, go into the Node Network to find the atmosphere. Connect the atmosphere's output to the input of the cloud's node. The output coming out of the cloud's node needs to go into the Planet object's Atmosphere Shader input.
If the TGC already has an atmosphere, then simply select the output from the lowest node in the clip file's node structure and connect it to the Planet object's Atmosphere Shader input. After that, the default atmosphere (that you aren't using anymore) can be deleted...or whatever.
If this doesn't make sense, consider going to the beginner tutorials to learn more about TG2. Hope this helps.
Hey there are some really cool clouds in here, but I feel *really really* dumb.
Once I download my file, how do I use it?
Where is the option to use clouds as per the attached file?
(it is late and I worked over 9 hrs today- brain dumb)