You can add information to the silkscreen layers. Use the text and drawing tools to add information to the Silkscreen top and Silkscreen bottom layers. If you already have text on your layout and need to move it to the silkscreen layer mouse over the text and press the p key to open the properties. Locate the layer drop down and select Silkscreen top or Silkscreen bottom. Remember if you select Silkscreen bottom you will want to mirror the text so it will be legible once the board is fabricated.
If you need to edit the silkscreen for a footprint you can do so using the footprint editor. If the part is already on your layout right click on the footprint and select edit footprint. If the footprint is not yet on your layout, you can select design from the menu bar and select edit footprint.