Hello,
The plus sign is next to fields when you click on the Set Visible Fields button to bring up the Select Columns pop up window. In this window is where you would see the plus sign next to some fields.

Steps to see:

1. Enter project customization from within a logged in project
2. Select Groups and Permission on the left side
3. Select a user group (pick one you have added, not one of the default ones)
4. Select the data hiding tab on the right top
5. Pick a module, such as say Defects
6. On far right will be filter and under it the list of entity names. Next to top is the Set Visible Fields button. Shows table image with arrow on it.
7. This opens the Select Columns window and here, to the right for the column/entity names, you can see a plus sign next to some fields (example Defect ID).