How are you trying to enter them? Go to your profile and at the bottom of the list of your tags should be a "edit tags and tag comments. Click that and go to the bottom of the page where there's an add tag section. Enter the tag name and a comment and then click the Enter New Tag button. If that doesn't work give us some more details where things go wrong. I think you may be required these days to enter a comment with each new tag, but it's a good idea even if it isn't required.