Here is a temporary work-around for the following bug, which prevents using the extension manager in nightly builds on Linux:
https://issues.slicer.org/view.php?id=4544
in the expanded Slicer directory, copy the contents of resources/ to libexec. Then restart Slicer.
root@kali:~/Downloads/Slicer-4.9.0-2018-05-02-linux-amd64# cp resources/* libexec/
root@kali:~/Downloads/Slicer-4.9.0-2018-05-02-linux-amd64# ./Slicer