Are you loading the client validation wizard?
When I load the wizard, I can see all of the rules applied to this page.
Perhaps try deleting the cache file from the users configuration folder:
common_installation_issues.pdf
there is another way to see the rules that are applied to the form, use the behaviors panel
in code view, select the form element, then go to Window -> Behaviors. this will show all of the behaviors applied to the form element, you can add validation by clicking the plus button and selecting WA Validation Toolkit -> Apply Client validations.
the problem wit hthe rule you added is that the list does not contain a blank value.
change:
<select name="paying" id="paying">
<option>Choose</option>
<option value="Visa">Visa</option>
<option value="MasterCard">MasterCard</option>
<option value="Cheque">Cheque</option>
<option value="Cash">Cash</option>
</select>
to:
<select name="paying" id="paying">
<option value="">Choose</option>
<option value="Visa">Visa</option>
<option value="MasterCard">MasterCard</option>
<option value="Cheque">Cheque</option>
<option value="Cash">Cash</option>
</select>