okay i got it to work now.
I just needed to zip "io_scene_valvesource" folder (like you said) and install it individually

@REE3.
What Blender edition are you using? I have file name "blender-2.80-893fa598319e-win64" from 2/25/19 download date.
I used your method and the installation is an success but check box nor description is visible in the Add-ons list when searched for

alright i'll report it to them. Also i downloaded a new Blender file today. And it says "Upgrade to 2.8x required"
(picture below)
https://cdn.discordapp.com/attachments/545304646831308810/545381321913008169/Capture.PNG

Blender 2.8 Beta can easily find BlenderSourceTool-master.zip when looking for Add-ons. But after installed, it gives off an error.
(picture of error below)
https://cdn.discordapp.com/attachments/538073972072251392/545038678897066011/Capture.PNG

And just yesterday installing the BlenderSourceTool-master.zip file was good but it did not pop-up in the list of Add-ons extendsion.