C:\Program Files (x86)\Steam\steamapps\common\sins of a solar empire rebellion
Double click "Sins of a Solar Empire Rebellion Dev"
When you start your game push Ctrl + Shift + . <- period
Then Click "A" [Change Galaxy]
Select your name
Then Click "F" [Spawn Entity]
Then Click "A" [Titan] (1 titan per click, must have a planet selected)
Choose however many of which ever titan you want. You'll go negative fleet and command crew but they will all work.
If you want to level up those titans instantly just to be that much more of a douche to the AI...
Go back to [Change Galaxy]
Click "C" [Change Entity]
Click "F" Give Experience (1 level per click, must have a ship selected)
If you want to learn about all of the commands click to your hearts desire, or read this guide made by GoaFan77
Galaxy Forge is a map making tool. So you'll have to make or alter a map and spawn titans before you even launch the game for them to show up. If you want to mess around with it any way...
C:\Program Files (x86)\Steam\steamapps\common\sins of a solar empire rebellion\Galaxy Forge
Double click "Galaxy Forge"