Hmm the dll file should be included with the zip, did you extract all of the contents?
Re-downloaded and extracted again and it worked no problem :) Not sure what was going on the first time.
I had a lot of fun with this, level design pacing and difficulty curve was excellent. Great ending sequence as well! My only very minor gripe was figuring out the controls. At first I did not even realize crouching and knocking on walls was an option. Would have been great to have either a tutorial level or tool tips.
Awesome game :)