Which browser are you using? What operating system? What are you trying to paste? If it's text and Windows you can type CTRL+V and paste. (Or right click your mouse and hit paste.)
If you're trying to paste an image or other file you'll need to follow the instructions for adding an image - keep in mind that the toolbar doesn't display in most browsers other than Internet Explorer or Firefox.
You can also try clicking the 'Bypass "Custom" Menu...' button (which should show up in your right-click menu for CR4); this should give you the normal right-click menu functionality.
CTRL+V should work no matter what and is a good shortcut for pasting. CTRL+C is the shortcut for copy.