Any reference to unofficial game modifications is strictly forbidden.
To download the DDDK, open up the main steam window (Library). Under the 'All Games' drop down menu, select 'Tools()'. Find 'Dungeon Defenders Development Kit' and right click and select Install, or just double click. Follow any prompts to download and install. If you want a detailed walkthrough for doing this, then just follow this link, Install Guide with Pics.
Once installed the DDDK will show up in your steam game list but will throw an error. We can hope Trendy will fix this in the future but for the time being you have to run DunDefTool.exe manually. By default it is located at 'C:\Program Files\Steam\steamapps\common\Dungeon Defenders\Binaries'. If you want even more information, then the DDDK Usage Guide should provide plenty of that.
Useful video or text tutorials are linked here.
UDK Tutorials Playlist
Cr4zy's Minimaps and Billboards
Cr4zy's Enemy spawning, setting up waves and wave sizes
Cr4zy's Treasure Chests, more mana and how to spawn them!
The DDDK was first released on November 11th, 2011 as a DLC. With the 7.40 patch it is now a separate download. Before that, you could not uninstall it without uninstalling Steam too.