Can a flyer who is standing on ground level terrain, make a hth attack against a figure in an adjacent square if that adjacent square is elevated terrain??

No.
Flyers only ignore the effects of elevated terrain for movement purposes.

Well I don't know what hth means, but can't flyers who are soaring make attacks against elevated terrain? or is it just other soaring figures? I always figured you could.

hth = hand to hand. aka close combat

Ok, that's what I thought but wasn't quite sure.

hth=hand-to-hand=close combat

Grounded fliers can make cc attacks against other grounded targets, soaring fliers can make cc attacks against other soaring targets. The exception is a power or other ability that lets it attack different elevations/flying levels specifically (like Leap/Climb or Divebomb).

Keep in mind though that a divebombing figure can only attack grounded figures and NOT figures on elevated terrain...

True, true...I didn't want to get too far into things past the original question.

As it's been said, a grounded flyer can not make a close combat attack against a figure in an elevater terrain.