If you are interested in using Photoshop extension files, such as the AD Impastoshop- Thick Paint Machine by Alex Dukal for example, and you are using Adobe Photoshop CC2019 or newer, then we would recommend that you use the Anastasiy Extension Manager to install your extensions. you can check out a free download link here: http://install.anastasiy.com/.
Once you have launched Anastsaiy Extension Manager, click on the Install button in the top left corner of the extension manager. From here, you can then search in your download section for the files you're looking to work with in Photoshop. Once you have found your desired file, click install and you should have installed your extension.
Once you have installed your extension, simply restart Photoshop and you should now find your extension under the Window drop-down menu at the top of the screen.
We hope that you enjoy working with your new Photoshop Extensions, however, if you are having any trouble getting up and running, please pop us an email and we will be happy to help!