is it flat?
How thick?

Make a slab or set of walls in the plan view as deep as necessary in the shape you require.

Make a 3D view - a parallel elevation projection from 90 degrees.
It is like turning the flat thing up on its edge.

See 3D projection settings to set this up.

With the 3D view looking like the top view of the element as you'd place it in the plan,

Go to:
Libraries and Objects> Save 3D Model As: Archicad Object File.

Save to a folder that you include in your Archicad library.
For the first one, you'll want to create a new folder and include it in your library load using Library Manager. (This can be confusing since the Archicad 10 library - the default load - is in a format that prevents additions. See library manager dialog - a new folder is needed)

There's lts of thinigs you can do to tilt and further modulate this object, but they all require GDL.

I would also mention the Cigraph add-on ArchiForma as a way to make objects that are inherantly tilting and editable.

I attach an illustration of this process. It should be used when you are making objects that are discrete and used a lot - say, if you might need to count them....

However, if you are merely using this shape once or twice, I suggest investigating the Custom Profile wall method, instead.