Here's my step-by-step guide with pictures how to get the eight Steam Achievements in the UDK - Unreal Development Kit. It's not that easy to make this happen. Some steps are needed and you can't overview your Steam Achievement status quo in your library, just see simple overview via the game library[abload.de] or in-game via this[abload.de].

It took me a while to create this guide and I hope you have success. Give me some feedback if you have troubles with my guide. :)

Important facts:
-> You don't need to install Unreal Tournament 3:Black Edition (unless you wanna play with UT:BE maps -> *read step 10 for more details) to get the UDK Steam Achievements; the "extern" UDK (about 1,8gb) is enough.
-> The UDK via Steam (library/tools) is not working for this (error: this game is not available at the moment, blah...)
-> Being on the map (step 8) you have to maximize the game window via Alt+Enter to successfully leave the map later. (step 9)


Here we go:

1) Download the UDK extern via this link[www.unrealengine.com]! Get the November beta one.
2) After installation, let the UDK start for the first time automatically. Finish the UDK.
3) Go to Start/All programs/Unreal Development Kit/UDK-2012-10 and then click on UDK Editor. You're in the Editor (NOT ingame) now.
Additional notice: Create a shortcut of the editor and put it in your task bar to get this started faster. :)
4) Now you have to go to File/Open (here[www.abload.de]) and for example to have a working map take the VCTF-Necroplois.udk one (or DM-Deck.udk)
5) Start the map through the edtor; this lasts some seconds.
6) Now you just see a picture in the foreground (you're STILL in the editor; NOT ingame).
7) Go to Play/On PC! (here[www.abload.de]) Seconds past by (for me about two(!) minutes!); saving/compiling the map and starts then - finally IN-GAME.
8) IMPORTANT: You have to maximize the game/the map with Alt+Enter to exit the map later again (view step 9) --> Thanks Chrissyx for this hint! :)
9) Problem: The map runs but you haven't got any enemies (bots) in your game. Exit the game with the ESC button - not the whole game/UDK - and you return to the main menu.
10) Go to Instant Action/Settings/Bot Skill level (for example: Novice)/Back/start game (take the VCTF-Necroplois or the DM desk map) *You can also choose more maps if you have installed UT3:BE.
11) And now you have "activated" the Steam Achievements.
12) Have fun collecting eight new Steam Achievements. :)

Maybe there is another easier way to "activate" the Steam Achievements in the UDK. If yes, let me know and I can re-edit my guide or "enhance" it.

Now have fun and grab these achies, hunters. ;-)

Greets
SgtFopper