How to use a plugin in eclipse?

I've downloaded many plugins, and 99% of them don't have any documentation on how to use it. The only thing they mention is to download it under your plugins directory. and then what? how do you get the "new" functionality working on eclipse..?
For instance, I downloaded the classloader plugin, which is A tool to help java developers define classpath references in the Eclipse IDE. Extends the statndard autocompletion and import system, to allow autoinclusion of classes that are completely missing from the current classpath.
[i downloaded this since I have never been able to use ant inside eclipse, i can only use it on command line]
I installed the plugin but have no idea what to do next?
any suggestions?

BTW, I was also unable to install the JfaceDBC plugin. Eclipse says the plugin is installed but it is not enabled. when I try clicking the "enable now" button, eclipse complains:
Graphical Editing Framework (2.1.2): Feature requires plug-in "org.eclipse.platform(2.1.2)", or compatible
I've also downloaded the GEF from here (the sdk, as well as runtime), but I still get the same error..
when I check the status of GEF 2.1.2 it says "the feature is configured properly".