[Garan]

You are correct that they go under the /plugins folder. The folder structure should look something like:

/plugins
..../author1
........abcdef.plugin
........abcdeg.plugin
......../abcdef
............somefiles.lua
......../abcdeg
............someotherfiles.lua
..../author2
............ghijkl.plugin
......../ghijkl
............somemorefiles.lua

In the above example, you would have 3 plugins from 2 authors - it is expected that each author will keep their plugins in a single subfolder of /Plugins to keep things organized. The first two plugins would be from author1 and the third from author2. The .plugin files that describe the plugin, version and path to the main .lua file must reside in a sub-folder of the /Plugin folder (the author's folder). It is then customary for the actual .lua files to be in a sub-folder under the author's folder.

In the case of Tonic Bars, there is a "tonicbars" folder level in the zip file that is extraneous. You have to extract the Tonic folder that is inside the "tonicbars" folder to your /Plugins folder, so the Tonic Bars folder structure should look like:
/Plugins
..../Tonic
........TonicBars.plugin
......../TonicBars
............somefiles
......../UI
............somemorefiles
......../Utils
............somemorefiles

That should set you on the right path to figuring out the correct folder layout for your other plugins.