The Tree Editor behaviour was slightly different in Forest 3.8, compared with the current version. Please, try this procedure:
1) Create the Forest object clicking one time.
2) Open the Modify panel->Geometry, add the HQ Plants models from the library. Be sure the default "template" item, is deleted or replaced at end.
3) Open Tree Editor, switch to sub-object level and select the tree (it should be only one).
4) Click "Reset Size", so it gets the size of the HQ Plant defined in the Geomety List, not the size of the default template.
5) From here, add new trees as usual.