Hi,

Welcome to Muvizu!

The sitting/standing position is related to the initial state.

If you want your character to start in the sitting position, select the character, then go to Prepare>Animation. Click on the Initial state>Stand button and change the icon from standing to sitting.

If, however, you want your character to begin standing and then sit down later in your animation, you need to animate this. When you are ready for your character to sit, go to Direct>Animation, then hit the record button (it looks like a clapper board) then select the sit anitmation button.
It's on page 3.

If you are struggling with Muvizu, it's worth while to spend some time watching the tutorials.
This one deals with animation:



Hope that helps!