OK, I moved the main LR window over to a side, hit the "synchronize" button, and, lo and behold, the dialog box appears on my SECOND MONITOR, which I had turned off. Fiddling with the 1. and 2. monitor controls in the lower left of the LR screen did not succeed in getting it to behave.
Any suggestions? (Come on all you guru's, give me a hand, please!)
Jack Winberg