It looks like your latest catalog was not on the external drive. It would be on the external drive only if you had either created a backup and restored in 'custom location' or moved the catalog folder manually from the Explorer.
So, you should do a search for files named 'catalog.pse11db' from your backups. If you find several instances, check the latest one. You can copy or move the whole catalog folder (which contains the catalog.pse11db database file) where you want.
Can you find such folders?
Actually I found the catalog (should have searched the forums first. ) I restored the catalog to my external hard drive. I then went to the Organizer in PSE 15 and tried to Manage Catalogs so that I could convert it. I selected custom location and pointed it to the folder. The program would not recognize it as a floder with the catalog data. So I copied the folder to ProgramData/Adobe/Elements Organizer/Catalogs where the default My Catalog is. When I tried to do Manage Catalogs at that location it still would not recognize it. I then renamed the My Catalog that was there and renamed my old catalog My Catalog and it still would not recognize it as a catalog folder. Help!!
Once you have found the catalog folder, you can't open it without converting it.
So, you should use the catalog manager and use the 'convert' button and browse to the location of the catalog to convert. Is that what you tried to do?