It seems to me, that the link for installing new plugins in that lesson (Back-end Developer Workshop/Configuring Your Development Environment)
http://apache.mirrors.lucidnetworks.net/sling/eclipse
is not very useful. It does not contain AEM Developer Tools, just only Sling IDE Tools! You need to install AEM Developer Tools from somewhere else.
I suppose it would be better to use the following link
https://eclipse.adobe.com/aem/dev-tools/
Sincerely,
Misha