The tree wall tiles are 128x128, so you're not going to be able to paint them on with a terrain brush unless the map is setup to be 128x128. If you're also placing trees and rocks and stuff, you're probably using a map set at 16x16. To my knowledge, this incongruity is not something Tiled is prepared to handle. If you want to use the tree wall on the same map as the rest of the tileset, you need to set the map to 16x16, then load the tree wall set at 128x128. Make a bottom layer for just the tree wall, and place each 128x128 tile manually, lining them up appropriately.