The pi-store owners no longer exist (brexit fun)
I was looking for it myself (using an old beginners guide) and had to do some searching and finally found it was closed. ironic when you consider there is a page on here that talks about yet no where on the top it says it is closed.
Anyways fear not there is a sub, located in your setting (preference).
Go to the add/remove program tab and up comes a new GUI interface which has available apps in categories and is also searchable.
Touch an app and the dialog box shows you what it is
Check the box next to it and click OK to install (much easier then sudo apt-get)
https://www.raspberrypi.org/blog/latest ... an-update/
rough video (I will make better one day)
Page that should be modified
https://www.raspberrypi.org/blog/introd ... -pi-store/
Just trying to help