/b t Tree doesnt place tree, just turns dirt into grass

[oryuuk]

Hey! Im very new to VoxelSniper, I know the basics but I need some help with the trees.
I have turned a large portion of a Desert Biome to Grass in my Server's spawn area and I would like to place natural trees around the area to make it look more natural.

When I do the command /b t Tree it shows a little menu with "tree" highlighted (see screenshot - http://prntscr.com/omflfd)

When I try to place a tree down it fills the dirt block to grass like it would with /b e fill.
Am I doing something wrong or is it something else?
Thank you for reading :P