To my understanding, KMS keys can only be used with KMS hosts. If you want to manually activate you'll need to use a MAK key instead.
So I come to a machine that I've got Office 2010 installed on and put in my KMS key. I've downloaded Office from my licencing site, (Office 2010 ProPlus with SP1) the clicked on the key link to get my key and entered it and Office tells me that he key is "Not a Valid Office Product Key".
Now this is the same key I've used to activate my KMS host (The KMS host is even giving me the last 5 digits of the key). I've checked for typo's, copy and pasted the key and so an and still the client PC is having none of it. I've checked the edition and it says it's a KMS volume client edition.
I know it can't activate until I've made 5 activations to the KMS host but it still should accept the key so it can try and activate.