1. I can't figure out how to add trees to my maps, I tried a tutorial on this and I get an error everytime I did it that way. I actually tried to use a model but when I run over it I pick it up.

2. I also can't figure out how to make clip brushes (invisible solids), there must be something simple that I am missing.

Thanks

for a model use env_model and browse to model.
http://collective.valve-erc.com/index.php?ent=env_model&game=dod

then clip it off so you can't run thru it.
http://www.mardymouse.co.uk/modules.php?name=Content&pa=showpage&pid=34

you got zoner's tools
http://collective.valve-erc.com/index.php?go=mhlt

to make clip brushes just make a brush and apply the texture called 'clip' to it. its definately in halflife.wad

Thanks Whiskas.

1 & 2 : Use the Search function to download the model_zoo.