In order for the multiple Title and Caption options to be enabled in the Popups tab, a "multiple selection" must be activated.

This can be done in a few ways:

1. If you have made a selection of images, then select a Popup Type, this will trigger a multiple selection.
2. If you have selected an image by clicking on it's name, then add subsequant images by clicking their checkboxes, this will trigger a multiple selection (in the Popup tabs and the Image tab)
3. Clicking the Insert Multiple Images button in the toolbar on the right of the dialog will trigger a multiple selection.