How do you access animations?

He probably just unpacked and extracted them.

I've never done it with TS4, but I used to do it with TS3. If you have a mesh with a skeleton, then you can simply animate stuff outside of the game. Loading and playing an animation is basically as easy as loading and playing a sound effect.

I've not used this Machinima thing as you can simply write your own code to load animations and call them at will, etc.

Technically-speaking, you can re-pack replacement/additional animation into the game - I've never tried it, even with TS3, as I was more interested in taking the animations out and using them in my own 3D environment.

This is proably what is used:

http://sims.wikia.com/wiki/SimPE#SimPE
http://modthesims.info/d/563256
etc

Normally, you unpack game media then have to convert the file formats to something you can work with.

So, getting into an archive doesn't guarantee you can simply work with the media straight away, without also converting it.