I had LR5.6 as a program on my MacBook Pro 10.8.
Downloaded LR/PS Creative Cloud.
I now have what seems to be a NEW version of LR in CC.
The OLD LR5.6 is GONE with all my collections.
Kindly advise how to retrieve all my old collections.
When you say you have no collections, do you mean that you 1) have no photos in your library at all or do you mean 2) that you have photos (under the Folders section of the Library), but nothing under the Collections section of the Library? As far as I understand, collections are stored inside of the LR Database so if the re-installed LR 5.6 opened an older copy of the database or created a new database, then you might experience things missing. Assuming this is the case, search your machine for a filename of the form "Lightroom 5 Catalog.lrcat". Mine is located in a subfolder in the Pictures/Lightroom folder -- your location might be different. If you can find the correct version of the file and open it with LR, then I'm guessing your problem will be resolved. The standard LR 5.6 is the same as the LR CC 5.6 so I wouldn't expect there to be any problems with reading the database.
Hope this helps,