New Widgets Not Showing Up

I’ve been using WordPress for a long time. For the first time, I recently have tried to install two 3rd party widgets:

WP-Mail
Subscription Options

I uploaded them to my wp-content/plug-ins folder but they don’t show up in the dashboard as ‘Available’. I thought perhaps the ‘Theme’ was the issue, but I switched to the Default theme and it makes no difference.

What am I doing wrong?

TIA,

—JC