Can I assume "The Sound of Music" and "Tintypes", etc. are records in the portal and not values in the value list?
If they are records in the portal you will need a dedicated field in that table for the checkbox to set. I'd make my value list with the single value "1" and name it "One" . (You'll probably need to use it again for some other checkbox in the future)
Put your new field in the portal and set it to a checkbox with value list "One" and shrink it down so only the check box is visible.
If "Tintypes", etc. are values in the value list, you're going to have to rethink your structure.
If this didn't help, maybe you could provide more information on how your tables are structured and how the portal was done.
Shoot. They are values in the value list. Is there any way to rescue this? I need a means to select multiple "Use in Shows" boxes (potentially hundreds), but I can't make check boxes scroll. That's why I thought a portal might be the best solution.
Why can't there just be a scroll bar with check boxes!?
What do you recommend?
Check out this file: https://www.dropbox.com/s/ti8vep64h67mtj2/CheckboxWScrollBar.fp7
It's in the older file format, but users with FileMaker 12 or newer can open it using Open from FileMaker's file menu to get a copy of the file that is converted over to the newer file format.