Minecraft servers are console or text-based applications. This means they don't present a visual interface for you to use, instead they have a scrolling window of text and a textbox to enter commands in.
To access your server's console, login to Multicraft at http://mc.hosthorde.com and select Console from the left sidebar. In the top textbox at the top, you can enter commands. Press enter or click send to run them.
When using commands, make sure to not include a proceeding forward slash. You'll often see commands written including the forward slash, for example the command to make a player an operator is /op Notch. However in the console, you would just enter op Notch and click send.