Combining imported meshes into one mesh?

I’ve created and exported a file from Skecthup into Unity as dae(collada) file, and It is made up of so many children that the game play lags horrendously. How could I mesh every thing together so it’s all one mesh on a game object, not loads of separate meshes parented. I have seen the mesh.CombineMeshes reference but I’m not sure if that’s actually what I’m looking for here, but I maybe wrong. thanks for any answers.

Sounds like you need to cleanup your model. You need to watch how many faces you have in your model. You might need to merge un-needed faces to lighten your model.
check out this primer on game development:
3D Primer for Game Developers:An over view of 3D modeling in games

Look at the section on polycount and optimization.