I tried to install a plugin : https://sourceforge.net/p/sweethome3d/p ... f-4.5.sh3p
I downloaded it and installed in .eteks/sweethome3d/plugins, I can see it under a tab "tools" but it is grey and I cannot activate it.

You have to select a room object before the option is available. The Roof Generator uses a room as a base to generate the roof. Without a selected room the option stays grey.

I tried to import the texture toitureArdoiseBleue.zip, and the anwer was that the new texture was available in the texture catalogue, but I can't see any texture catalogue.

You see the texture catalog when you modify furniture and click the button to set a texture. This can be the texture button in the "Color and texture" section or the texture button when you choose materials and the modify button.