Doug,
There are several parts to the question.
Version 5.0
1) Is there a method to view/compare more than two images at a time?
No.
2) There is a button above the thumbnail panel- "Show Selected Thumbnails". I cannot find a way to select more than one image to use this feature.
Select more than one image before you go into the Preview and you'll see the selected set instead of all thumbnails in the thumbnail strip.
3) +/- buttons on top menu bar. How is this feature used? I see no way add images to a "Set". What is a set? Is this in the help files anywhere?
When you have two or more selected images you have a selected set. The plus button will add an unselected photo to the selected set. You can use the two icons in the thumbnail strip to toggle between all thumbnails and selected thumbnails.
The minus button will remove the selected status of a thumbnail thus removing it from the selected set.
HTH,
-Kirk