Your expression box is most likely sitting in a section bound to a node higher up the tree than your text box. So, definitely, the expression box would use the XPath my:projectLead and your text box would use ../my:projectLead. The ../ is taking you up the tree - so let's say your text box is bound to a field under the myFields folder called my:leadName - you'd need to go up to myFields (../) then to the project lead field (my:projectLead), giving you ../my:projectLead.
Now, as to why the value isn't showing in your text box - well, it should. Your text box isn't also bound to the my:projectLead field, is it? Can you attach your template to the Options tab so I can take a quick peek at it?