When I place my trees I can walk through them I don’t know how to use the collisions
First off you ned to check that there is any collision on the mesh to begin with, If you double click the static mesh object to open it up. then from the collisions tab at the top of the page you can choose to view either the simple or complex collision.
Simple collision is a basic hull object like a box or capsule that covers the tree. its simple shape means it’s less accurate in terms of physics but a lot cheaper performance wise.
Complex collision uses the actual mesh as the collision object but for something like a tree it could end up being VERY expensive.
So, if you show simple collision and there is none, you can generate it yourself from the collision menu.
If however you need a bit more accuracy with the collisions then it may be worth creating your own custom UCX collision and importing it along with the static mesh.
Hope this helps.