ep-5nva sli user's manual

That is not too helpful. What exactly is your problem?

where can I download this manual?

EPoX went out of business a couple of years back so you cannot approach them, and a quick search of the usual suspects such as MOBOT has not returned anything other than a spec sheet here.

Sorry, looks like you are out of luck unless you can find a fellow owner who still has access to a manual.

Alternatively, what problems are you having that you need the manual to help with? It may be that DaniWeb members can help without the specific manual being required.