Go to Releases and download the ZIP file from the latest release. The one that looks like quickadd-x.x.x.zip.
This ZIP file should be extracted in your Obsidian plugins folder. If you don't know where that is, you can go to Community Plugins inside Obsidian. There is a folder icon on the right of Installed Plugins. Click that and it opens your plugins folder.
Extract the contents of the ZIP file there.
Now you should have a folder in plugins called 'quickadd' containing a main.js file, manifest.json file, and a styles.css file.