Drag'n'Drop to install is in fact the right (and hassle free) way to install extensions in both Chrome and Chromium, but it's disabled by default. You can do it that way:
Open a terminal (Ctrl + Alt + T) and start your browser with this flag:
chromium-browser --enable-extensions for Chromium
google-chrome --enable-extensions for Chrome.
When your browser starts, navigate to
Drag the extension file (
*.crx) to the extension tab. A small text window will indicate
drop to install or something similar. Drop the file by releasing the mouse button and click
yes when asked if you wish to install the extension.