Where is Qt designer app on Mac + Anaconda?

I expect it's Qt Creator that you should be looking for. Note here: -

the integration of Qt Designer under Qt Creator is first mentioned at least as early as Qt 4.7 (ca. late 2011)

Qt Creator includes a code editor and integrates Qt Designer for designing and building graphical user interfaces (GUIs) from Qt widgets.

If it's not in your distribution, you can download it separately here.

You can try open -a Designer from your terminal to launch Qt Designer that comes with Anaconda (version 4.x).

If you have Qt5.x, you may want to launch a newer version of Designer by open -a Designer-qt5.