I am unable to see my cookbooks in an older version of MasterCook.
Please tell Windows to show you hidden files and folders on the computer as explained here: Show Hidden Files/Folders
Then use Windows to search for one of your cookbooks by its name. Are you able to locate them that way? If they are anywhere on your computer, Windows will show you where they are when you search for them this way.
Please make sure you physically go to the folder where it finds these most recent cookbook files and copy both the .mc2 and .mcx files to back them up. See the first part of this article: Backing Up Files in Version 14 or Less
You can merge your files into MasterCook 15 as explained here: MasterCook 15 Merge