getElementById doesn't work for RSFormPro in Joomla 1.5

I am working on an RSFormPro project with a javascript function to populate a dynamic drop down list based on the current date and time. The drop down list function is triggered at page load time. The problem is that the id referenced by getElementById is null at load time. I have tried both

window.onload=generateoptions(document.getElementById('trainingSession')); 

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and

window.addEvent('domready', function() {
generateoptions(document.getElementById('trainingSession'));
});

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Here is a link to the problem.

I  have opened a support ticket with RSJoomla (the RSFormPro developer) but due to time zone differences am seeking help here.

Are there any experts with RSFormPro experience that can help me?

Thank you!!