The general navigation in Proloquo is done via tabs. Every different colored column contains buttons which belong together. The tab above the column opens more buttons which belong in that category.
Sometimes there are more words available than can fit on the page. In this case, you can browse to the next page and back using the arrows in the bottom-right corner.
If you need to go back to the home screen, you can always do that by tapping the Home button in the upper-left corner.