From within your WebCT course, click on the Chat link. The link may exist within the course or in the course toolbar.
Depending upon how your instructor has configured the tool, you may see a chat room or a combination of a chatroom and whiteboard. Click on the appropriate chat room.
The room will open in a new window. Enter your text in the bottom text box and click Send when ready. If your instructor has enabled handraise mode, you will need to click "Raise Your Hand" in order to enter text.