I am working at a hospital environment. Since the hospital environment is internal, the 3d slicer cannot access to Internet. Therefore, 3D Slicer cannot download and consume extension immediately.
Slicer applications and all extensions are fully portable. You can download and install Slicer and extensions on any computer that has internet access, then copy the entire installation folder to another computer and everything will work.
Alternatively, you can use the new extension download site as described here.
It works. You may need to use a recent Slicer Preview Release. If you have any problems then describe what you did, what you expected to happen, and what happened instead.
This is a known issue, you can track the progress of fixing the problem here. You can use the new extension download site as described here.
We are in a hospital environment and encountered issues installing extensions. Working with IT security, it turned out a problem with the corporate firewall (this is explained by IT to me. Review of the traffic to slicer-packages.kitware.com indicated that it was hitting the SSL inspection of the traffic. The site appears to be breaking due to the SSL inspection). They ended up implementing a SSL inspection bypass, which fixed the problem for us.
This may not be your issue, but at some point you may need to get your IT team involved.