In Synaptic, when I click File
> Add downloaded packages
and navigate to the folder with the debian packages, the debian packages are greyed out, which means I cannot select them.

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2 Answers
According to this question you are misusing this feature. Apparently, this feature in Synaptic is not for installing individual DEB package files.
To do this - without using the command line - navigate to the folder where the packages are and double-click one (or more?) of them. This will bring up the Package Installer and you should be good to go.

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ok it's better than the dpkg -i ... – maniat1k Feb 25 '12 at 02:31
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Will this work if I need to install a set of packages from files? (E.g., accountsservice and libaccountsservice0. As I needed in that case: https://bugs.launchpad.net/ubuntu/+source/accountsservice/+bug/952909/comments/9 ) – imz -- Ivan Zakharyaschev Jun 15 '12 at 21:48
complementing @Mei answer: https://askubuntu.com/a/107521/46437
as an alternative to accomplish the final goal.
search for: Debian/Ubuntu: Making a Package Repository on Your LAN
(best shot, has the commands) How to create a local APT repository?
http://freesoftwaremagazine.com/articles/debianubuntu_making_package_repository_your_lan/
https://www.maketecheasier.com/setup-local-repository-ubuntu/
this way is easier when there is a lot of dependencies like trying to install yakuake on ubuntu 18.04 :)

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