SoftChalk can open only zip packages that it created. If your zip file was created manually or by some other software product, then SoftChalk will not be able to open it.
You zip file must also have the same name as your lesson. For example, if your lesson is named Jefferson, then the zip file must be named Jefferson.zip. Sometimes when you upload your zip file into a Learning Management System, it might rename your zip file. For example, it might rename it Jefferson1.zip. In this case, you would need to rename the zip file to Jefferson.zip (the original zip file name).
If you aren't sure what the zip file was named originally, open the zip package manually and view the contents. By viewing the contents, you should be able to determine the name of the lesson – look at the names of the html files contained within the zip package.