I've just installed W8 as a test and here is what I have (or am doing) to get these 3rd party add-ons to install.
- Using IE downloaded Calibre (used defaults in IE), ensured I selected SAVE option
- Installed calibre
- Download 3rd party add-ons (used defaults in IE), ensured I selected SAVE option
- Clicked "gear" symbol in IE and selected "View Downloads" (Ctrl J)
- Clicked the "Open" button adjacent to the zip file downloaded
- Screen now shows the contents of the zip file just downloaded
- Drag the Calibre_Plugins entry to one of the locations shown on the left hand side of the screen; I selected Downloads listed under Favorites
- In calibre, go to Preferences | Plugins | Load plugin from file
- In the directory selection box displayed, click on the same location as you extracted the directory to (so in my case Downloads listed under Favorites and then double-click on Calibre_Plugins to open that folder
- Finally select which plugin you want to install