Hi,
Firstly you need to load the object into ArchiCAD so you can use it.
Create a new folder on your machine somewhere, and then extract the contents of the zip file you downloaded into it. Any other library parts you download or create can also be put into this folder in future.
Go into ArchiCAD, then open the Library Manager and click 'Add', then navigate to the folder you just created. Select just the folder and click 'Choose' and ArchiCAD will load the contents.
If you now go to the window tool, the freeform window will be available to select.
You place the window like any normal window. The window will be square shaped to start off with, but when you select it a number of hotspots on the corners and midpoints will be visible. In elevation or in the 3d window, Click and drag on the midpoints to create new corner nodes. Drag any corner node onto another corner node to delete it. You may find it more accurate to this editing in an elevation so you can draw some temporary lines to snap to if the shape get complicated.
Have Fun!