How to make this model solid?

Hey everyone, due to the fact a lot of source engine models lack physic objects and are not solid, I’ve decided to try and make at least some of them solid so I can better use them in-game. I’m using Blender for this and I’ve added a pic of what the model looks like. It would be much appreciated if someone could point me in the right direction as of what I need to do to solidify these models, this one in particular:


Thanks in advance.

Here’s pretty much all the info you need.

First one is general info and how to make one, second is how to compile it.

Okay I’ll try that now, thanks.

[editline]29th October 2013[/editline]

Is there an easy way I can create a collision mesh for the tree? I’m not experienced in modelling so I apologise if I sound inexperienced. However, I did manage to make a simple collision mesh by just smoothing whatever was there by default but as you could imagine, it was really dodgy and was just a big invisible blob of solid space rather than a collision mesh that followed the shape of the actual tree. Any ideas?