I recently moved my Lightroom catalog from my PC to my new MacBook Pro, so I think I can answer that one. My catalog file is about 1GB big, spanning 111GB of RAW files + 10GB of JPEG files.

The really good news (at least to me) is that you can actually use your catalog as is on both machines. What I mean by that is:

• your catalog file is binary compatible on both PC and Mac (it's a SQLite database internally),
• you can open your catalog with either your PC or your Mac,
• you can work on the same catalog file back and forth on either your PC on your Mac,
• your pictures do not have to be at the same location.

The way this works (from my understanding), is that the location of your photos (RAW, JPEG, etc) is stored in the catalog relative to a root location. This root location is a setting kept internally on either your PC or Mac, not in the catalog itself; as long as you specify your root location, all your files are found relative to it, on either machines.

To be more specific. On my PC, all my photos and catalog are stored like this:

• D:\users\barre\Pictures\RAW
• D:\users\barre\Pictures\JPEG
• D:\users\barre\Pictures\Lightroom\Lightroom 4 Catalog.lrcat

In LR's catalog, a photo is not a reference to D:\users\barre\Pictures\RAW\2011\2011-08-12\IMG12.DNG for example, but stored as 2011\2011-08-12\IMG12.DNG relative to the a root folder, here D:\users\barre\Pictures\RAW. As you can see, as long as your root folder is known, everything else is found. In the case above, I actually have two root folders, RAW and JPEG (i.e. the set of unique top-most folders).

I connected my MacBook Pro to my PC using an Ethernet cable and copied the files over; this is the fastest way to transfer this huge amount of files from one to the other (see here and here).

On my Mac, I ended storing my photos like this:

• /Users/barre/Pictures/RAW
• /Users/barre/Pictures/JPEG
• /Users/barre/Pictures/Lightroom/Lightroom 4 Catalog.lrcat

Note that although they kinda look similar, they are really two different style of paths (no disk drive, different slashes on the Mac, etc). I just chose to keep a similar folder hierarchy, but you don't have to.

At this point you should probably make a backup of your catalog (if you haven't already) and store it somewhere while you experiment.

I launched Lightroom on my Mac (your serial number for LR on your PC will work too by the way), and pointed Lightroom at the catalog file mentioned above: Lightroom 4 Catalog.lrcat. The first time you do so, LR will not be able to find your photos, it will display questions marks in the left panel next to your root directories:

In my case, it was unable to find the RAW and JPEG folders, which are the top-most parents of my collection. No worries, just right click on the root folder(s), select "Find Missing Folder", then point at the new locations. Here, I pointed at /Users/barre/Pictures/RAW and /Users/barre/Pictures/JPEG. Lo and behold, LR will now reconnect all your photos.

The beauty of this (and a testament to the talent of Adobe Engineers in my opinion), is that you can work on your catalog on your Mac, then copy that catalog over to your PC where your photos also are, keep working on that catalog, then pass it back to your Mac, etc. The first time around, it might also complain about missing folders on your PC, just do the same as described above for your Mac (reconnecting RAW to D:\users\barre\Pictures\RAW for example).

Why would you do that? Well, for example, I have a desktop PC at home that has a bigger screen, but I also have a Mac laptop where I edit photos when I travel. Syncing your catalog and photos between both computers is an entirely whole discussion, you know of services like Dropbox or Google Drive, but I've been using Unison for years and mentioned it in more details in another SE Question. Unison only transfers the difference between both files and is very efficient at it; not going to lie, it's not easy to setup but well worth the trouble. In practice, a modification I make to my 1GB catalog file on my Mac means only a few KB are transferred to my PC when sync is performed, very quickly. Syncing my catalog and my collection of photos between my PC and Mac is not only very easy, but provides backups at the same time.

Hope this helps