I am having problems building a working magma forge in my fortress (using the most current version of DF). I have built magma forges in prior versions and am aware that I need to place the "darker green" x in the magma forge template over the hole leading to magma in the Z level below the forge. When I build a forge in my current fortress it apparently builds ok, but when I try to set jobs for the completed forge it says it needs magma under the forge. I have excavated a large open area (approx. 50x40 tiles) one Z level lower in my fortress that is still filling with magma (currently filled to 2-3 depth) from a connected magma pipe. Do I need to wait until the magma fills to a higher level than 2-3, or are there other requirements I am missing to get the forge to work?
As indicated in the second paragraph of the introduction to the Magma Forge on the DF Wiki, you need to wait until one of the eight non-center tiles of the forge fill to a depth of at least 4/7.
To build a magma forge, you must have magma-safe materials (non-magma-safe materials will not be shown on the material list), an anvil, and a build site where at least one tile directly beneath one of its eight non-center tiles contains magma at a depth of 4/7 or greater.
Assuming your influx of magma is sufficient to overcome the evaporation process, you ought to be able to wait for a while and the forge will become active.