The serial number that you use for installtion on PCs is not the same for Macs.
You have to contact the Adobe support to get a seriel number for installations on Macs.
You can download the installation file here: Adobe - Lightroom : For Macintosh : Adobe Photoshop Lightroom 5.5
Thanks very much Axel, regards Jude
Actually your serial number will work for both the Mac and PC versions. There are not separate serial numbers for the Mac and PC versions. However, you need a version 5 serial number. If you paid for an upgrade and got a second serial number (you had one already for version 4) you will need both serial numbers. Download the current Mac version of LR. It will ask you for your serial number. You will enter your version 5 upgrade serial number. Then it will ask you for your previous serial number and you enter that one. Then you will be good to go. The reason you need two serial numbers is that you paid a discounted price for your upgrade. You need to prove that you purchased an original version (any earlier version) of LR at some point.
Moving LR to a new system requires more than just moving the pictures and LR catalog. For a full-description of what needs to be moved over and how to do it:
Pay particular attention to step #2 above and the files that need to be copied and moved over Click here for a full list of all the Lightroom files. Folder locations are different on a Mac than PC. I just did this after a hard drive crash using a backup copy of all my files including those at:
Windows—C: \ Users \ [your username] \ AppData \ Roaming \ Adobe \ Lightroom \
Windows—C: \ Users \ [your username] \ AppData \ Roaming \ Adobe \ CameraRaw \
It took a while until I had all of my custom Develop presets, templates, etc. copied back to proper folder locations and plugins reinstalled. Leave LR installed on your PC until you have verified that the new Mac installation has everything moved over.
As far as I can tell there are no shortcuts or tools available to simplify the process. I would love to see Adobe create a migration tool that would copy ALL of the required files including custom presets, templates, lens profiles, etc. to a backup image, which could then be used to "automatically" reinstall them inside LR on a new system. This image file could also be used to "backup" all of the required files to restore a LR installation in the case of a hard drive failure. The 'Store presets with this catalog' Preferences option does NOT contain all of the items that you will need and in fact creates a potential issue since "new" presets created are no longer stored at the above User folder locations.