It's like placing water. You place the asset block, then stretch it out to the size you want using the yellow tags, then you double click it to bring up the menu on the RHS and choose the assets and density etc...
Don't be surprised if it crashes the game sometimes when you first place it and/or if you try to copy and paste. Keep trying if it does, it only does it when under load.
When I double click the asset block, the trees disappear when I hover over the left flyout. I also don't know how to place water, but I will look at the manual.
"The trouble with a one track mind is it's narrow gauge and lack of switching controls." - Buzz Baxter