How to do add a multiple choice question in a designed slide?

Not sure what you mean when you say you've designed the slide already, but I'll tell you how I do it. I'm not a big fan of the assessment questions with the radio buttons, so I design a page and insert my own custom buttons for my answers. In this case, I'd have a custom button for True & one for False. Type your question in a text box.

Then go to the Insert menu and click "Convert to Freeform". This will open up a box asking you which type of question you'd like. For a true/false you can select "Pick One" as the learner would only pick one of the answers. This will take you to the normal question building screen but instead of typing in your answers, you simply select the page element and indicate which one is the correct answer.

Hope this helps!

Thanks Jamie, that´s perfect, it is exactly what I needed. Eleonora

Years past, still such helpful. many thanks Jamie

still helpful...don't know if you will see this, but thanks.

So glad to see that this conversation is continuing to help folks :) 

Thanks for chiming in to share, Harlan. Welcome to E-Learning Heroes too!