If it is a textfield, try changing your code to use .val() instead of .text().
$('#m1-myapp-multiPage1-page1-text1').val("Hello World");
I have always referenced them directly, the catch here of course would be to have unique names for all the textfields on your pages.
$('#m1-myapp-text1').val("Hello World");
EDIT: Since your subjects states that you are using a label and not a textfield, then try assigning the value directly like this:
$('#m1-myapp-text1').text("Hello World");