Hi
Here is an alternate solution. You can try this

1.    Open the form in design mode
2.    Go to properties window of the text box by double clicking on it
3.    Click on “Display” tab
4.    Click on “Conditional formatting” button
5.    Click on “Add” button
6.    In the first drop down select the field’s name and in second dropdown
select “is not blank”
7.    Check “Read-only” check box
8.    Click “Ok” three times
9.    Save the changes made in the form

I hope this helps you