How to use an XML element in an XML function

I know that generating fixed with text from XMl is a common need. And I know this question has been asked before, but every answer I've found is unclear to me due to my lack of understand of XML functions.

I have several examples of using an XML function to pad a string to a particular width. But none of the examples I'f found are very clear on how to use these functions.

I need to take the contents of an XML element, and pad that to a fixed with with spaces. I have found such functions at the below links.

http://ift.tt/1AI4lua

XSL left-right justification with Padding

http://ift.tt/1zDZpF5

But in each example they pass in a string. How do I use such functions by passing an element such as 1234 Street