This problem appears to be an issue only with paying members, so I combed through every single paying member's profile. Every one of you that has reached top cat status should be able to change your custom title now. To change your custom user title:
1. Click User Cp near the top left of the page.
2. Click Edit Profile from the left table.
3. Scroll down to see the custom user title box.
4. Enter the text you want.
5. Scroll down to the bottom of the page.
6. Click the Save Changes button.
If you still can't see the custom user title box, pm me or reply here.