Ashenvale Piers...

I have seen on a map, they used the "Ashenvale Pier" model to use as a pier, I am not sure how to make this walkable, how is this possible?

Have you actually tried using the Ashenvale Pier doodad?
Obviously not, since if you had, you'd know that it's walkable by default.

Yeah, there walkable, but what I wanted is for it to be able to be placed on half water, half land, like a real pier, how can I do that?

Well Either make the half water part shallow water. Or make the units flying when they are over the water.

Or use invisible platforms.

Use Ctrl+PgUp, or Ctrl+PgDn to move the Pier up and down until it's at the height you want.

lol, I just walk straight through it though.
EDIT: Like, It wont walk on top of it, it just walks straight into it.