Hello Mr Farley and welcome to Moodle!
I'm sorry to hear that you keep getting an error message when trying to import IMS Common Cartridge files. I guess it's because Moodle currently only supports the import of IMS CC version 1.0 and 1.1 packages, and packages from Schoology are probably 1.2 or 1.3. (MoodleCloud is the same as standard Moodle when it comes to IMS Common Cartridge import and export.)
If you have time, please create a tracker account and vote for Moodle supporting IMS CC v1.2 and 1.3 - MDL-60803.
Otherwise, you may know that you can rename your IMS CC packages from .imscc to .zip in order to open them and retrieve files.
Regarding how to add your classes, as Ken mentions, we have documentation. The Admin quick guide is a good place to start, or you could check out the links provided in your MoodleCloud welcome emails.