It pains me to hear people pronouncing "achtung" incorrectly. Yes, even Larry Mullen Jr.
Firstly, there is NO "k" or "c" sound in achtung. It is not "Acting Baby" as my sister likes to call it. "Ach" is a predominantly German sound, though you'll hear it in Afrikaans, Dutch, Flemish, etc. If you've never heard it before, it can be a little difficult to imagine in your head. LivLuvAndBootlegMusic's definition is good, though - a little akin to a cat hissing. My German prof explained it quite well too. Say "human" and hold onto that h sound.
Secondly, you do not usually pronounce the "a" as in "apple." It's more of a hard "u" sound, as in "underhand." I do believe it varies according to what area of Germany you're in, though.
The "tung" is little like "tongue." You just have to lower the "u" sound a little.
And thus concludes our German lesson for the evening. Next week, we shall discuss the correct pronunciation of "Vorsprung durch Technik."
U2 fans hear it on Zooropa pronounced correctly by Bono, yet still manage to arse it up when they talk about it.