You seem to be misunderstanding how plugins work - downloading, extracting, installing and then using them...
First of all - permissions.
To be able to add files to your Sketchup Plugins folder you need to have its security-permissions set to 'FULL'.
By default Windows limits it to read-only [i.e. no 'write'] because the folder is inside the 'Program Files' folder.
To fix this select the Plugins folder icon, right-click > context-menu > Properties > Security > Edit...
Give everyone listed FULL permission.
OK to apply/save changes.
You will need admin-rights to be able to do this...
So now you can add things to the Plugins folder.
Plugins come in several forms - single .rb/.rbs files and for more complex files archives containing typically a.rb loader file and a subfolder of 'helper' files helper files - these are ZIP/RBZ archives. When extracted they need to go into the Plugins folder in the same relationship as they have in the archive - i.e. all files must stay inside their subfolder [if any] when added to the Plugins folder.
Some tools require you to have also downloaded and installed a folder of 'helper' files - usually called a 'Lib'[rary] - Fredo and Thomthom do this the most. A few other tools also need a helper, obtained from an external url - like 'progressbar.rb' from Smustard.com... These 'dependencies' are clearly explained in each tool's page.
Now for the good news... SketchUcation have recently launched a 'Plugins Store' that includes most of the Plugins that are available on the forum - to use it within SketchUp [v8M2] you'll need to download and install it from an RBZ archive - but that is clearly explained on its page... Once installed the tool allows you to search through hundreds of available Plugins, by keyword/category/author, it includes descriptions/links-off the more info... etc AND best of all it has an Auto-Install button... So provided that you have sorted out your Plugins folder's permissions to FULL, it will download/install/load the chosen Plugin[s] for you... See this page viewtopic.php?p=467479#p467479