Open your favorite browser and find video you like on YouTube. Then select URL in the address bar and copy it to the clipboard.
Now switch back to YouTube to MP3 Converter and press the Paste url. Video will be added.
Copy URL in browser like in previous way. Switch to the program and press the Gear icon at the bottom left. Select Add multiple urls and window will appear. There you can paste saved url. Edit it manually, add more url or paste the list of url. After editing press OK and all of them will be added.
Another intuitive way is drag and drop of the URL. Just drag the URL from browser and drop on the window of YouTube to MP3 Converter. And video will be added too.
There is an option for adding URLs automatically. To enable this feature press the Gear icon at the bottom left. Select Auto add from clipboard. And since this time there is no need to press additional button and switch to converter to add video. Just browse through YouTube and copy video URLs to clipboard. Program will detect them and add to the list.
Another way is accessible on Mac OS X only. You may drag video URL from browser and drop it on the YouTube to MP3 dock icon. The most convenient in this method is that there is no need for the program to be launched. Just keep the icon on dock and drop URL any time you need.