Your components are not up-to-date, you should consider updating Designer to latest version.
Within your infinit loop you call USBUART_Start() repeatedly, this should be called only once
You did not call USBUART_Init()
There is a simple example within the USBUART-datasheet, stick to that.
You are presumingly using MAC-OS, I do not know if the required driver on the PC-side (emulating a COM-file) is functional.