Revert to your main presentation, keeping your poster window open.
Select your title slid and click on the text box containing the title. It now becomes highlighted
Go to the edit menu and copy.
Go back to your poster window and now paste from the edit menu.
The title text box will now appear.
Position the box in the centre top of the page.
Now choose your font colour and size.
Now stretch the box to the edge of the page.