semaso - most of the plugins are made by 3rd party developers not Scirra. Rexrainbow, the creator of the rex_board plugin you are asking about, made all of them for C2 and C3 however not all of them work with the new C3 Runtime. (The newest fastest runtime available). You can see which runtime your project is currently set to by checking out the properties bar on the left hand side (with nothing currently selected) and then press "View" next to "Project Properties". In the new bar open the area called "Advanced" and in the "Runtime" Dropdown, try setting that to "Construct 2", and see if the plugin now shows up.
I'm guessing you have the C3 (Construct 2 runtime) version installed, and not an updated C3 (C3 runtime) version.