I have friends who play harp, and I've never heard of a lever that makes a srring flat, only sharp. If you want an E-flat, then sharp a D string.

Folk harpers tend to prefer music in C, D, G and A, the sharp keys. If a note comes along that doesn't fit those keys, they turn a lever to play that note, then turn the lever back to normal to finish the song. Some people don't even use the lever, they just sharp a string by pushing on it with one finger while plucking with another.

I think that you either have an unusual harp or you need to read your owner's manual through a couple times.