A warning before you go out and buy the game

[Chirke]

When trying to activate your copy on Steam, keep in mind that some codes of the full retail game only activate the game's multiplayer part and not the single player and Steam's activation list is straight up misleading people since it doesn't state this anywhere.

[Jito463]

I think it's more likely you did something wrong, or something got goofed up during the activation. DM uses Source, and is a Steamworks title. There is no reason the SP side of the game would not activate on Steam. I'd check with support.

*Addendum*
Just as I was about to post this, I checked the FAQ page in the support section for Dark Messiah. It's odd, but it may fix your problem.

https://support.steampowered.com/kb_article.php?ref=8497-TVMI-4261

Invalid CD Key error

If you consistently receive an Invalid CD Key error when trying to register Dark Messiah of Might & Magic, try re-entering the CD Key backwards, digit for digit. If you continue to have difficulty, please contact Steam Support.

[Chirke]

There is no reason the SP side of the game would not activate on Steam.

This is exactly what I thought as well. So I tried the backwards thing first. No luck. Then I contacted support with the problem and an image of my retail copy. They told me it's perfectly natural since my copy must be some cursed version and I should deal with it. Needless to say, it's pretty frustrating, given how their list said I can activate my game. You can see it for yourself:
http://i56.tinypic.com/nn337q.jpg

I also told them to stop misleading people at the very least, but that part was just ignored.

[IcarusNine]

There ARE multiplayer-only keys. A magazine was distributing a few of them. If it's an error, it's not likely to be a usual error.

Though, it is strange that there'd be an MP-only retail version. Well, unfortunate, not strange. I could see why they'd do it if the publisher (especially the ever famously evil Ubisoft) offers a discount for MP-only CD keys to resellers who want to sell bargain games. Thing is, if the retail copy you bought says it's only multiplayer then you're out of luck.

[Chirke]

No, the copy is perfectly normal and contains the single player.

[TempS]

why buy the retail one anyways? its $9.99 already on steam.... lol that's dirt cheap enough considering what a great game it is. and if you see this post before the sale on it ends tomorrow, it's only $2.50 =)