I want to migrate my existing Lightroom 3 library, which is on a PC, onto Lightroom 4 on a MAC.I recognise that I will need to buy a new MAC Lightroom licence but how do I go about the transfer ? will the existing library structure of files be maintained?
Lightroom is a cross-platform license, so your serial number and install disks will work on both Mac and Windows. No need to buy a new license as long as you're only running on up to two of your personal computers.
Catalogs are also cross-platform. You should be able to transfer your catalog and images to your mac.
Because the directory structures of Mac and Win are different, you may need to locate missing files at a high level after you open the catalog on your mac: