As far as I know, there is no order to the list in the Model Summary table of the variables used in the table. It merely lists those which are significant, and which have been used in build to the tree. Whilst the CRT method gives you the Importance to Model table, there is no equivalent for CHAID (or Exhaustive CHAID).
You could pick the variables used in the top levels of the tree and use regression to assess the importance using the standardised beta values. You could use enter or stepwise methods.