Ayman:
To place the stair at angle, use the second Geometry Method (Rotated) in the Info Box. When placing the stair, you can use the Relative Construction Methods (in the Control Box palette) to align the stair perpendicular to the edge of the slab. Or you could just place the stair and use the Relative Construction Methods to rotate the stair afterwards.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David