By default all videos are downloaded to your Videos folder (C:\Users\<Your PC Name>\Videos\RealTimes\RealDownloader) in your PC library. If you've made changes to them, you can find where you've set RealDownloader to download it to.
1. Open RealDownloader
2. Click RealDownloader on the top left corner and click Preferences
3. You will find the location of where you files are saved in "Save local files to:"
Alternately, you can right click on a downloaded video in your RealPlayer library, right click on it and click Locate File
Please note: We have no idea what your stored location would be if you made any changes.