If you install Eric (specifically Eric 4) from the Ubuntu software repos, the Eric plugin repository points to a location that's unavailable:
I'm sure there's a way to change it but I don't see how to do it in the app itself (haven't started poking around to see if there are config files somewhere yet), but luckily Eric plugins are just zip files so you can download them from a repository URL that works, and then add them to Eric.
The working plugin repository is here:
From there just do a ctrl-F to find the plugin you're looking for, then copy/paste the URL for the plugin's zip file into your browser (or use wget or whatever floats your boat) to download the plugin.
With the zip file downloaded, in Eric go to Plugins -> Install Plugins, click Add, and then point to the zip file you downloaded.
If someone knows how to change the plugin repository URL in Eric I'd love to learn.