To Install the headset app on the Apple Vision Pro's app store all you have to do is go to the downloads page in the headset's browser, then click on the app store icon at the bottom of the page.
NOTE: ExpandXR is unlisted in the app store, meaning you'll need the link to access the app page, you won't be able to find it by searching the name.
The installation process is very easy, it’s like installing any app on the Apple Store. Select the app and press Get. Once it has downloaded you’ll see it in the home menu.
As with any Apple Vision Pro app, just look at it and tap your thumb and index finger to launch the app.