The collections are saved inside the catalog that you are using in Lightroom
You have to physically copy the catalog or export the collection as catalog.
You can right click on the collection and choose export as catalog.
then you can import the same catalog in the imac.
Export a collection as a catalog
You can export a collection of photos as a new catalog. When you create a catalog from a collection of photos, the settings in the photos are exported to the new catalog.
Exporting a smart collection as a catalog adds the photos in the smart collection to the new catalog. It does not export the rules or criteria that constitute the smart collection.
- Select the collection or smart collection that you want to use to create a catalog.
- Right-click (Windows) or Control-click (Mac OS) the collection name and choose Export This Collection As A Catalog.
- Specify the name, location, and other options for the catalog, and then click Save (Windows) orExport Catalog (Mac OS).
Import photos from a different Lightroom catalog
When you import photos from a different Lightroom catalog into the current catalog, you can specify options to handle new photos and photos that already appear in the current catalog.
Catalogs from earlier versions of Lightroom, including public beta releases, must be opened and updated before they can be imported into another catalog. See Upgrade a catalog from an earlier version of Lightroom.
- Choose File > Import From Another Catalog.
- Navigate to the catalog you want to import and click Open (Windows) or Choose (Mac OS).
- Specify the photos to import:
- Under Catalog Contents, make sure the folders containing the photos to import are selected.
- To select or deselect individual photos, click Show Preview, and click the box in the upper-left corner of any preview image. You can also choose Check All or Uncheck All.
- In the New Photos area, choose an option from the File Handling menu:Add New Photos To Catalog Without Moving
Imports photos at their current location.
Copy New Photos To A New Location And Import
Imports photos to a new location. Click Choose and specify the folder.
Don’t Import New Photos
Only photos that exist in the current catalog are imported. Lightroom determines a photo is a duplicate (already in the catalog) if it has the same, original filename; the same Exif capture date and time; and the same file size.
- In the Changed Existing Photos area, do any of the following:
- Replace metadata, Develop settings, and negative files to override all the settings in the current catalog. If you choose this option, you can select the Preserve Old Settings As A Virtual Copy option to keep a backup. You can also select the Replace Non-Raw Files Only option to avoid replacing raw negatives. If changes to raw negative files affect only metadata, selecting this option helps save time.
- Replace metadata and Develop settings only to leave the negative files (the source photos) unchanged. If you choose this option, you can select the Preserve Old Settings As A Virtual Copy option to keep a backup.
- Replace nothing to import only new photos.
- If photos in the current catalog are missing and can be found in the imported catalog, indicate whether you want to update the metadata and Develop settings for these files. Select the Preserve Old Settings As A Virtual Copy option to keep a backup. If the photos missing in the current catalog appear in the imported catalog, specify whether the missing files are copied and where they copied to.
- Click Import.