how to render using all the cpu cores?

It depends on the software package you are using for rendering.

For Maya you can specify it under the instances Template settings.

For Lightwave renders you have to specify it in the LW config files that muster reads (LW10-64.CFG, RenderThreads #)

3ds Max and other renders might have other places for setting the number of threads that is available for use.
Sometimes you can set it inside the program itself so it´s scene file specific or it might be a separate config.
Google a little about the software you are using and ill bet you can find where to specify it.

And you should avoid using the Realtime Priority since the render engine might completely overwhelm the instance and you might lose connection to it or make it unresponsive. High is fine

Also note that different render engines use threads in different ways. some processes during the render phase might be limited to a single thread. so you will see a variation of cpu usage during the render period.

Muster only reports the current cpu usage of the instance since it supports a multitude of software packages and every engine is programmed and works in different ways.

If you want so take a closer look on cpu usage during different phases a tip is to activate the live render log in muster and login remotely on on of the instances so you can see the cpu usage first hand directly on the instance itself that way you can figure out what´s going on. Maybe an old unoptimized plugin is limiting threads usage who knows

Scene file loading is usually a single threaded process in most renders for instance.

You can also think about that the total render time is “loading time + calculations” so if you can change something in the scene or upgrade plugins, drives, network and so on you can might be able to shave of a few min per frame if your loading times are high.

Good luck