
validation-toolkit/
and under the How TOs click on the Applying CSS styles to form elements with Validation Toolkit (PDF) pdf. This will explain how to add validation to an input field. You could skip the css styling portion of the document.
Let me know if you have any questions.
Note:
In the document it says Select WA Validation Toolkit > Server Validations. from the server behaviors panel, but in your case, you should already have a validation toolkit entry under your server behaviors panel. So you could just double click it to add your input field.

After reading a link you provided i did figure out how to do that validation( not to have an empty field).... but i could not find a single example on how to check if user has already registered his name ....
On my reg form I have another field called a UserName .... and when they register i d like to be able to validate if that user name is already used in my sql database by someone else...
please help ..... let me know which validation i should use ....
thanks