Getting nice tree models in your game can usually be done with one of three methods:
1. Have an artist make them for you.
2. Learn how to use a 3D modeler and make them yourself.
3. Buy/find pre-made royalty-free trees that you can use.
The trees from the screenshots you refer to were downloaded from this website:
I imported the sample trees into Milkshape, and exported as Ogre .mesh. I don't include them in the PagedGeometry examples due to licensing issues - they're not my property to redistribute, especially in an open-source package.
And, if you use any tools, how to export the model to *.mesh?
You need to find an Ogre .mesh exporter for the modeler you use. For example, if you use XSI to model your trees, you'll need to download the XSI exporter and install it. You can get most of the main exporters here:
And even more here:
You can search tools to make trees, I mean special tools just fro trees, like ngPlant.