Login to Admin Mode and go to the page which you want to add the content to (by clicking that page's link in the navigation). Once you are on the correct page, hover your mouse over the "Page Menu:" box or link and the Page Menu will appear. This gives you options for managing pages.

Hover over the "Add to This Page" option on the Page Menu to see the different types of content "blocks" you can add to the page, such as a headline block, an image block, a WYSIWYG block, a slideshow, a photo gallery, etc. The most common and flexible type of block is the WYSIWYG block. Selecting the type of block you want to add will add a new block of that type to the bottom of the current page.

Enter the desired content into that new block and then click the [Save Changes] button.

For types of content, refer to the "What types of content can be added to a page?" entry.

To move the new block to a different location on the page, refer to the "How do a change the order of blocks on a page" entry.