I'm afraid, that at the moment, there's no Arnold material version included in The3dGarden packages (as mentioned in the description on the product page: "Materials included for V-Ray 3.0 and above and Corona Render 1.6 and higher.
") and as we are pretty busy these days with other pending tasks, I'm afraid I can't give you a time frame when that might happen. As you can imagine we already support a lot of render engines and we have only limited resources, so we have to focus on those that are most popular. That said, as Arnold integration progresses and a critical mass of users would be reached, we'll certainly prioritize it. Unfortunately, at the moment there are no many vegetation libraries with the Arnold renderer support yet (also for this reason we've added the Starter Library with Arnold materials in the FP6). The most of available libraries on the market are currently using V-Ray materials.
You might find interesting and helpful the following "How to convert V-Ray materials to Arnold materials for rendering in 3ds Max
" Autodesk's article.
I hope that helps a bit.