I have just tried to install the extension and I cannot reproduce the problem. You can install it in any of the following locations:
.config/inkscape/extensions/ in your home folder.
In both cases, Inkscape shows the Sozi extras submenu in the Extensions menu.
I have tested with Inkscape 0.92 and Ubuntu 18.04.
I need to make a presentation with jpeg files
This extension allows to embed video and audio. You don't need it if you want to insert JPEG images (use the Import function in the File menu).