I added a custom monster, but somehow it doesn't show in Thngs Mode's list.

Call me stupid or blind, but I did the exact same thing as all videos around YouTube and for me it doesn't show, so basically I have downloaded Chainsaw Zombie as a custom monster for my custom wad, I did every step by doing it with Slade 3, copied and paste it on my wad and somehow Doom Builder 2 and GZDoom Builder wouldn't able to find the custom monster, once I have entered the game with my wad activated, once I typed on command "summon chainsawzombie" and it did spawn, can someone please explain to me, I'll screenshot if I have to, thank you.

In the DECORATE definition, does it have a number at the end of its actor name?

It should look like this:

Actor ChainsawZombie 10000

Where 10,000 is the DoomEdNum that allows you to place it in the editor.

You can also add a couple extra flags to make it appear in different categories, like this:

Actor ChainsawZombie 10000
{
 //$Category Monsters
}

I tried it didn't work, I'll send the decorate:

ACTOR ChainsawZombie 3204
{ 
    //$Category Decorate
    //$Title Chainsaw Zombie
    Health 30 
    Radius 20 
    Height 56 
    Speed 13 
    MeleeDamage 1 
    PainChance 200 
    MONSTER 
    +FLOORCLIP 
    Obituary "%o was sliced by a madman zombie with a chainsaw!" 
    SeeSound "grunt/sight" 
    MeleeSound "weapons/sawhit" 
    PainSound "grunt/pain" 
    DeathSound "grunt/death" 
    ActiveSound "grunt/active" 
    DropItem Chainsaw 
    States 
    { 
    Spawn: 
        CHAN A 4 A_Look 
        CHAN A 4 A_PlayWeaponSound("weapons/sawidle") 
        CHAN A 4 A_Look 
        CHAN B 4 A_PlayWeaponSound("weapons/sawidle") 
        CHAN B 4 A_Look 
        CHAN B 4 A_PlayWeaponSound("weapons/sawidle") 
        Loop 
    See: 
        CHAN A 4 A_Chase 
        CHAN A 4 A_Chase 
        CHAN B 0 A_PlayWeaponSound("weapons/sawidle") 
        CHAN B 4 A_Chase 
        CHAN B 4 A_Chase 
        CHAN C 0 A_PlayWeaponSound("weapons/sawidle") 
        CHAN C 4 A_Chase 
        CHAN C 4 A_Chase 
        CHAN D 0 A_PlayWeaponSound("weapons/sawidle") 
        CHAN D 4 A_Chase 
        CHAN D 4 A_Chase 
        CHAN A 0 A_PlayWeaponSound("weapons/sawidle") 
        Loop 
    Melee: 
        CHAN E 1 A_FaceTarget 
        CHAN F 3 A_MeleeAttack 
        Goto See 
    Pain: 
        CHAN G 5 
        CHAN G 3 A_Pain 
        Goto See 
    Death: 
        CHAN H 5 
        CHAN I 5 A_Scream 
        CHAN J 5 A_Fall 
        CHAN KLM 5 
        CHAN N -1 
        Stop 
    XDeath:
        CHAN O 5
        CHAN P 5 A_XScream
        CHAN Q 5 A_Fall
        CHAN RSTUV 5
        CHAN W -1
        Stop
    Raise: 
        CHAN NMLKJIH 5 
        Goto See 
    } 
}

At risk of stating the complete obvious and sounding like an asshole, did you check the bottom of the things list for the DECORATE category? That's where it should be, going by what I see here.

No you are not, still learning as you can see :D, and yes I did actually checked, and it doesn't even showing up at all even before I put the comment code "//$Category"

Try searching the number 3204 in the type field of the things list. If it doesn't come up (which it should), then something is radically wrong with either the actor or the editor.

Like here, where my arrow is, I typed "1" and I got all the things whose names have 1 in them somewhere, so you could type "Chainsaw" and it should find the weapon and your monster.

Below the list you can see "Type," which here is 51. If you type 3204 in that field, it will pull up the actor with that DoomEdNum, which should be your Chainsaw Zombie.

If it's been defined properly (If you got it on Realm667, there's the distinct possibility that it wasn't, which is infuriatingly common), then this method will find it for you.