All questions about Simple Multisit here...

"I am getting a "No room to sit here, try another spot." message when I am trying to sit on a prim that is configured to be seated for more than one avatar." How can I fix it?

You could fix this problem by adding other sit targets to prims that you already have in your link-set by copying and pasting the following script into them :

default {
    state_entry() {
    llSitTarget( <0,0,1.0>, ZERO_ROTATION );
    //llRemoveInventory( llGetScriptName() );
    }
}

After which you may delete the script from the prims

How come I'm not getting the setting menu when I sit on the ball ?

Please make sure that you have dropped the ball into the same prim that you dropped the setting script into and try again.