Blender mesh prim

Hi, I am new to Blender, downloaded it all fine, got the scripts to make sculpty meshs (Primstar and the extra ones) all works great, but... how on earth do I make a square prim? I have only 5 options, a flat square being one but I cant make it a cube... the other 4 options are a circle, a doenut sort of shape, and... oh an empty can! and a half circle... no square cubes in thie 5 options.

How can I do this please?

Thanks