In short- You unzip the folder and your STL is inside.
The long version- In order to expedite the download process, the STL files are shipped to you in a compressed folder. As the name suggests, the compressed or zipped folder makes the file smaller and dramatically reduces your download time.
To uncompress or un-zip the folder:
PC- Right click on the folder that you have downloaded from CADspan and select "Extract" Follow the prompts and you will end up with an uncompressed folder containing your STL file.
MAC- Double-click the zip file and the file or folder will be decompressed into the same folder the compressed file is in. The new decompressed item will have the same name as the original file.