WORKAROUND #1:
Do not use drag and drop. Instead, take your cursor and click on the workflow line of where you want the next activity to go. That gives that part of the workflow the focus. Then, go to the activity toolbox and click once on the activity you want to add. This will add the activity just clicked to the location of the workflow that was clicked.
WORKAROUND #2:
Single click an activity in the toolbox and it will be added to the bottom. If you need to move, only drag the activity upwards, until it snaps to the correct location. The activities don't like to move downward.
STATUS:
Waiting for a fix to be released in a future version of Toad Data Point.