I had the same problem for a while. Was really discouraged, but finally got it, so I'm sure you can too.
Over the Pa map, Zoom in on the town you're working on. Then go up to "select era" (pull down). select 1937-42, my favorite since it shows the old grades best. When the red dots appear, click on one for the area you want. You can then quick zoom (+) on the thumbnail to get your bearing, and then go back and scroll down within that window to pick a resolution to download. I go with high res. since the older pics are a bit fuzzy. Keep trying, you'll get it.
Over the Pa map, Zoom in on the town you're working on. Then go up to "select era" (pull down). select 1937-42, my favorite since it shows the old grades best. When the red dots appear, click on one for the area you want. You can then quick zoom (+) on the thumbnail to get your bearing, and then go back and scroll down within that window to pick a resolution to download. I go with high res. since the older pics are a bit fuzzy. Keep trying, you'll get it.