When you log into webchat, you should see a "tab" in the top-left appear that says "#enigma" and then inside the window you should get a welcome message welcoming you to enigma. If you're not at this point, let me know. Otherwise, continue:
Next, you must click on the white box at the bottom of the page. A caret will appear, allowing you to type. Type something, then press the Enter key to submit your text. This is very similar to IM. Just notice that the textarea at the bottom of the page must have the focus (the caret must appear).
Hope this helps.