We use Adobe Illustrator CS2 but it's likely the same for CS.

1. Open the PDF from inside Illustrator.

2. On the program menu select Window, Workspace, [Default]. This should bring up several property editing windows and the tool bar. If the tool bar is not visible, go back to the menu and select Window, Tools. This should popup the toolbox window.

3. On the toolbox window you can click the Type Tool (looks like the letter T) and move your mose over the PDF where you wish to add content. Left-click and hold donwn the button. Then drag it and you will see a rectangle appear. Let the mouse button up when you have a nice-sized box. This is an area where you can type text to add. When done typing you can resize the box, change the font from up in the menus, and more.

Other tools allow you to do other things.

When done remember to Save.

Note: if your adding the text to an area "messes up" the underlyng content, click undo, then attempt to add this box on a clean area of the PDF. When done typing you can click the Arrow in the upper left corner of the Tool Bar. This will give you your normal cursor back. Then single left-click the box you created and use your mouse keys to move the box over the area you wish the text to appear.