To add text to your file, you may position the cursor at the point you would like to insert text and begin typing. EDT is automatically in insert mode, meaning existing text will be shifted to the right as you type. Pressing ENTER in the middle of a line breaks the line and moves the portion to the right of the cursor to the next line.
To add the contents of another file, you may use the line-mode INCLUDE command. Position your cursor at the place you intend to insert the external file and issue the line-mode command INCLUDE filename, where filename is the specification of the file you'd like to include.
To add text currently in your paste buffer (see, "Moving or Copying Text") move the cursor to the desired position and use PASTE (PF1,KP-6).