That was my answer as well, reported it.

unfortunately it is still neither accepted nor confirmed to be wrong.

Three years later and it's still not fixed??? :O "чем" is the instrumental case of "что", so yeah, it should be accepted.

Also my answer..

i would also like to know why this is чем over как

I don't think that they are equal as Чем in this case actually mean 'With what' (can I help you). While Как would mean 'How' (can I help you).

Как is actually more apropriate

I am wondering the same. It is actually more polite to use "may" instead of "can". Is there some other way to say this in Russian, or is this just a variation that DL hasn't added for this translation yet?

С чем = you're doing something alongside something else. Я ем суп с ложкой = I am eating soup and my spoon is chillin' alongside me (or it's a ... spoon soup, doesn't sound too tasty). By itself without a preposition it means you are using that thing as your tool/instrument.

Чем in the case is With What Как is How

That would mean "what [tool] can I use to help you?" I don't think the Russian means that.

That would require чем-то