Are there any tutorials on how to use those weapons and monsters found at repository?
Awkwardly stupid question indeed, but I couldn't find any tutorial/help after hours of searching.

"Use" how? If you mean running them in a game and seeing how they work, run the wad (drag-and-drop onto zdoom.exe, or right-click>open with), drop to the console (~) and hit "summon *monstername* ", for example, for monsters. For weapons, you can hit "Give All" (or any of the classic Doom cheats that give weapons), and "Use WeaponName* ".

If you mean adding them to your mod, you do that with a wad editor like Slade 3.

And it's actually a good question. I should put up something in the Tutorials forum for how to run these, or easily add them to a wad.

I meant how to install/adding them to my mod. But yeah, it might help newbies if there would be a tutorial on how to do this.

Anyway, I used the "Import multiple files to the archive" and imported the Autoshotgun Zombie into the wad.
I saved and went to Doom Builder, placed a thing, and tried to edit it but I couldn't find the zombie from the list of... things.

Either I did something wrong or then there's something else I need to do.

First of all, you can't have a WAD within a WAD. You've got the WAD "TEST" and inside it you have the WAD "AutoShotgunGuy". That won't work. However, what you want to do is open both WADs (the map and the monster) in Slade (Yeah, Slade supports having multiple archives opened in the same instance), click on the monster WAD tab, select all the lumps there (DECORATE, SNDINFO, etc..) and copy them, click on the map WAD tab and paste the stuff you just copied. After doing that, things should look somethiing like in the linked image:

http://i.imgur.com/6Rq7D.png (Image is too big to be displayed directly here)

Second of all, in order to see the monster (or any thing for that matter like weapons, ammo, etc..) in Doom Builder and use it in game when placed in the map, the monster needs a unique DoomEd number associated with it.

Alright, I got the autoshotgun zombie to work. Thanks.
In case I want to add multiple monsters and weapons (which is likely) is there anything special I need to do?

And as well, I installed a weapon which works but I can't switch to the custom weapon after I have switched another one, so, any help on this?

You need to assign it a weapon slot.