Normally, if you upload a SCORM 1.2 zip file when creating a unit, it will be automatically imported. However, there are cases that this will not happen, and the file will simply appear as available to download. Common reasons for this are:
The file is not SCORM 1.2 (but SCORM 2004 or some other standard)
The file is not packaged according to the SCORM 1.2 specification (very common case: The zip file contains a folder with all the files, rather than the files directly, as imposed by the standard). You can fix your file following these instructions.