Import files from another catalog

If you do not see the file handling options, then the explanation is as follows. The catalog that you try to import contains 'missing' images. That can happen if you copied that catalog (and its images) to another disk so you could connect that disk to your iMac. The catalog you want to import still links to the original location of the images, however. Because the images are 'missing' Lightroom cannot copy or move them on import, so that option isn't shown.

If the images are on the same computer then you don't need to move them. You simply want to add them to the other catalog. Images are not in the catalog. The catalog is simply a database that keeps track of where the images are located on the computer as well as the adjustments you have made to them and any collections you have created that contains those images.