I did that, hit build, and I can't seem to figure out how to get that custom widget into the designer.
What do you mean by "get that custom widget into the designer"? Do you mean:
  • Use the non-Qt widget in a Designer layout for another widget. You insert similar widget into the new design and use the Promotion feature to make it use your class instead. See Using Custom Widgets with Qt Designer
  • Make that custom widget appear in the widget tool box at the left of Designer. You need to create a plugin for Designer to enable this. See Creating Custom Widgets for Qt Designer