The client offered on this page is built using javascript and either websockets or (as a fallback if websockets are not enabled) flash. It may not work on some (mostly older) browsers, such as Internet Explorer 7 and below.
If the client doesn't load, first try reloading the page. If it still doesn't work, you can try enabling javascript (as well as flash), or use a different browser. Alternatively, use the Java Client, plain telnet, or download a more complete mud client elsewhere (for example Coldbeer, a client developed for Discworld). To connect to discworld on another client (or telnet), just connect to discworld.starturtle.net on port 4242 (or 23).