Welcome to the forums. I would suggest posting questions about the Revit plugin in the Revit sub-forum. You're bound to get quicker responses there.
The best way to work with trees in the plugin is to use proxies. The plugin allows us use a Revit family as a proxy for an external model and, as of recently, we can permanently associate that family with a particular proxy. So the 'aspen' tree family in Revit would always 'call' a certain OBJ or ORBX file and render it in the plugin.
There are many resources online for OBJ trees & plant, including Xfrog, Evermotion etc. Look at the manual for how to use families as proxies.
Hope that helps.
Win 8(64) | P9X79-E WS | i7-3930K | 32GB | GTX Titan & GTX 780Ti | SketchUP | Revit | Beta tester for Revit & Sketchup plugins for Octane