The current *beta* release is: .606b, for use with Warband version 1.160 (though 1.153 should work as well)

How to install Native Expansion:

Download Native Expansion right here.

The module is in .zip format. Your OS should support opening it without additional software.

Extract the Native Expansion .zip into the 'Modules' folder, located inside your M&B: Warband folder.
It will appear as 'Native Expansion [BETA]' in the drop-down menu when you launch M&B.

Steam Users: Your M&B: Warband folder is located under C:\Program Files\Steam\steamapps\common\mountblade warband\Modules
Steam users with 64-bit Windows: Your M&B: Warband folder is located under C:\Program Files(x86)\Steam\steamapps\common\mountblade warband\Modules

Additional Recommendations:
We highly suggest you also install the Siege Pack, an awesome addition by Lord Samuel that updates city and castle maps to have additional ladders for sieges.
It is located here: http://sami.is.free.fr/datas/temp/NE_Warband_Siege_Scenes_Pack.zip

soulmata personally suggests you use the M&B Battle Size Modifier, and crank your battles up to at least 400 if your CPU can take it - and 800 is even better.