First, click on the "Profile" link in the upper left:
Then click on "Edit Your Details":
Then look for the "Custom User Title" area:
Type your desired user title in the text box provided, then scroll all the way down and click on "Save Changes."
The ability to create your own user title may be linked to your total post count (too low = cannot create one), or it may be restricted for some other reason. I'll be getting in touch with PG admin to find out.