How to Fix OS X El Capitan Beta Updates Not Showing in Mac's Software Update

Whether you’re in the Public Beta program or the Developer Beta program, open the Terminal app and enter the appropriate command string from the choices below:

Public Beta tester? Set the OS X Public Beta catalog:

If you’re part of the Mac OS Public Beta testing program and want to set that as the appropriate software catalog stream, use this:

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11beta-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

Registered Mac Developer? Set the OS X Developer Beta catalog:

If you’re a registered Mac Developer and you want to set the Mac Developer Beta software catalog stream, use this:

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.11seed-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

Confirming the Software Update Catalog Change Worked

Once you have set the catalog to the appropriate URL, you can quickly verify the change was effective by using the softwareupdate command to list all available updates:

softwareupdate --list

You can then either return to the App Store and install the beta update as usual, or if you want to stay in the Terminal app you can install OS X updates from the command line too by specifying the package to install.

How to Reset to the Default OS X Software Update Catalog

If you messed up the syntax or otherwise just want to reset back to the default software update catalog stream, use the following command string:

softwareupdate --clear-catalog

Again, you will only need to do this if you attempted to refresh the App Store with Command + R and it did not work to display the available beta software update.

Source: droidOSX