The question comes to your mind why I need to install Chrome extensions manually where I can easily install any extension from Chrome extension gallery with a single click, because some times these extensions can fail to install by showing “Extensions Install Failure” errors and manual installation can also be helpful to install extensions from third party websites also rather than installing extensions directly from those websites.
Update: You may have actually come to read how to install extensions from outside web store, then read this
Manually installing Chrome extensions
1.Right click on the install button on the extension page and click “Save link as “
2.Save extension to your desktop.
3.Now if you have already 7-zip installed in your Computer,right click on extension saved to the desktop and extract its contents.
If 7-Zip not installed in your Computer then try to rename .crx extension to .ZIP and extract its contents using WinRAR.
4.Now open Google Chrome, type Chrome://extensions on Omnibox and press enter, expand Developer mode click “Load unpacked extension” and select extracted contents folder.
5. Extension for Chrome will be installed.
Update : Use Google Webstore Downloader extension to download Chrome extensions, then install them easily.