Although you can bind a key for it, the usual way to bind is to open the console and... :/
Yeah, you want to /bind x toggleconsole and then use x (or whatever else) to bring it up. To do this without being able to open the ingame console, you'll probably have to either:
1) start up tremulous with a command line/terminal, you should then be able to enter the command directly into the terminal window, or
2) find and edit your autogen.cfg file to include that command (leave out the slash): it should be set automatically next time you run tremulous.