I have just purchased my first Mac and am trying to access my Lightroom catalog and photos which are on an external hard drive. I was using a windows laptop with a external hard drive. I have downloaded the Lightroom program but can"t figure out how to access the catalog and photos. I am sure it is just simple commands that I have not mastered as a new Mac user. detailed instructions would be very helpful, thanks
You can either use File > Open and navigate to the catalog, or double-click the catalog file in the Finder to launch Lightroom with this catalog.
I don't use a Mac myself, but I've read that external disks that have been used on a Windows PC need to be formatted a certain way to work on a Mac. If that is the problem, somebody else will have to help you.
If you were using that external on a Windows computer it is more than likely formatted NTFS. OS X can Read that format but can't write to it natively.
You will need a utility that allows OS X to write to NTFS formatted drives. There are several of them. Some paid and some free.