There are a few reasons it might not work.

Try navigating to the page using a Watir browser running through 
Interactive Ruby so you can do some live experimentation.

Try using longer paths, like specifying any parent elements (frames in 
particular):
thisfield = 
$browser.frame(:index,0).text_field(:id,'IDITForm@pmlPercentage')

Ensure that the page is fully loaded before looking for the field, In 
order to do this, I use Watir-Webdriver rather than just Watir alone, so 
I tend to use when_present:
$browser.text_field(:id,'IDITForm@pmlPercentage').when_present.set(23)

Otherwise, try locating all (or some of) the text fields and iterating 
through them to make sure you have the details exactly right (name, id, 
value, etc.):
$browser.text_fields.each {|el| puts el.id }

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