transfer photo library from external hard drive to iMac

I copied my photo library from my MacBook to an external hard drive. It is titled 4-21-19.photoslibrary on the external hard drive. I would like to put that library on my new iMac. I plug the external hard drive into my iMac, go to the photos app then import to search for the library on the external drive but the 4-21-19.photoslibrary is greyed out. I can't import it. Anyone have any suggestions on how to import this library? My MacBook is 3 years old and I just purchased the iMac last night (not sure if this matters).