Hi, because Google always removes our Extensions for violating Youtube's policies, so we publishing source code and guide anypeople install Chrome Extension from source.
1. Download Youtube Downloader Extension and unzip it.
2. Go to chrome://extensions/ and check the box for Developer mode in the top right.
3. Click to button Load Unpacked in the top left then path to folder just unpacked in step 1 then click to open.
Then check in tab bar right if has icon Youtubedownloader is installed successfully.