There are easy ways, hard ways and there are safe ways. I suggest you use the 'safe way'. That way you won't mess something up in the process.
1. Launch Lr on your MacBook in Library Grid view
2. Select the files you want to transfer to the iMac
3. Choose Export as catalog from File menu
4. When dialog opens make sure you check the boxes as shown in screenshot I've included below. This will mean that the catalog includes only the photos you're selected, their previews and the negatives.
5. Copy the exported catalog and contents to the iMac (external hard drive, network, whatever)
6. On you iMac use the Import from Another Catalog command from File menu
7. Choose the catalog you just exported and copied across then configure the Import from Catalog dialog to ensure that the negatives are copied to the correct location on your iMac.
I did mention that there are easier and quicker ways, but I recommend you stick with above until you have more experience with how Lr handles the process.
HEy thanks a lot! That doesnt seem hard at all. I'll try it!
If I follow your instructions will I end up with just one (my original) catalogue? Also could you elaborate just a bit on that last part? - i.e. steps 6 and 7? thanks a lot!