You are looking for view entities, here an example from the manual:

ENTITY* compass_panel =
{
type = "compass.mdl";
layer = 2; // display above panels with layer 1
flags2 = SHOW; // visible on screen from the start x = 100; // place 100 quants ahead of the view
y = -50; // 50 to the right
z = 0; // and center vertically
}

EDIT: @Uhrwerk: Hehe, I was looking for that page. Btw. how to search the online manual?