You don't state specifically what macbook pro you have but from what I can tell from everymac.com you should be able to run OS X Yosemite 10.3.3. If that's the case then I'd update your OS so you can run the latest LR (which is 5.7.1). I if you are stuck on 10.5 then I think you will have to use LR 3 which you would have to get from ebay or something. Adobe won't sell anything but the latest version.
Yes as Joshua says, if you can find a copy of LR3 it will work with OSX 10.5 & 10.6
Yosemite takes a lot more power; so I would be careful about updating an older Macbook Pro although the enhanced RAM may help.
Thanks a lot guys!
As 99jon says, I think Yosemite will be too high for my late 2008 macbook pro, would you consider upgrading to e.g. Snow Leopard or Lion?
I do have a key (I ran LR 5.7 on a PC from work, but have to hand in that computer, since I quit my job). Would I be able to run that key to activate LR 3? On my Macbook?
Thank you for your answers!
Lightroom serial numbers are version-specific. A Lightroom 5 serial number will not work with Lightroom 3. You will have to find Lightroom 3 and purchase it in order to make it work.
You will not be able to use a LR 5 key on an older LR. While an intermediate version of OS X may be easier on your system and may allow a newer version of LR than LR3, if it is not current you still have to get that old version of LR from an alternate channel.
If you have an easy way to clone your system I would try upgrade you Yosemite and putting LR 5 on it since you have a key. If it doesn't perform to your satisfaction you can always clone back to 10.5 and seek an alternate solution.
However, was the LR 5 key owned by your previous job along with that other laptop? If so, it violates the license for you to use it.
I will consider your answer, to clone my system.
The LR5 key has always been my own proud possession, with my job knowing me using the program on company property.
Thank you for your kind suggestions,