Onyx make accurate and polygon-intense objects.
The file format you need in Archicad is a 3Ds. You may need to adjust the XYZ axes if the tree comes in sideways.
You will not render a forest of them, but use them for foreground effects.
It is an entire essay to explain how to work with Onyx and optimize them for Archicad.
Attached is a sample of a rendering used with Onyx.
Consider the Hoshino tress as a viable alternative. And Archicad 11 has some improved 3D trees in the library.
Dwight Atkinson