Open it from your Applications folder. It lives in the menu bar, not the Dock.
Then press Ctrl+R to open the wheel.
This installer places files outside your Applications folder, so dragging the app to the Trash does not remove all of it. A proper uninstaller is included — find it in Rapid Radial's menu-bar icon under Uninstall Rapid Radial…, or or find Uninstall Rapid Radial in your Applications folder:
Uninstall Rapid Radial
It asks whether to keep your settings. Unregister your licence from inside the app first — that cannot be done afterwards, and the seat stays used.