Here's where you have to install the dowels .
We started at center an measured every 16" drilled a 3/4" dia hole one the center of the edge of the board a little more than 1" deep. I then poured a little glue in to the hole and tap the dowel in to the bottom. The spacer in only needed during assembly. Once the table is screwed to the frame you can remove it.

Enlarged view