Once you are logged in to your Dashboard, the first application listed under "Chatbot Pro" will have a blue tab below it that says "go to chatbot pro". Clicking on it will take you to a page where a pop-up will appear in the middle of your screen showing the "GPTs".
The default setting in the dropdown menu is for the model engine "GPT-4o mini". You can simply click on the other model engines of your choice (such as ChatGPT-3.5 turbo or GPT-4o), as well as GEMINI and other LLM engines, to select them for use.