When you open your App Store, then the Purchased tab - do you see a line for "OS X El Capitan Beta"?
Click the download button on that line. It will take a few seconds before anything happens, then you will see a download progress bar.
Wait - it's more than 6GB, so will take a while. The installer will automatically open when the download is complete.
I suggest that you use one of the methods to create a bootable flash drive from that installer. Just search for that.
Or, use the good utility "DiskMakerX", which bypasses the terminal commands that you would otherwise need.
Then, you won't need to download the installer again, if things go south and you need to reinstall.