You can add a new stock in demo. I just tried.
Click the row in which you wnat to add the new stock in the column "Underlying" (in the default configuration it's the first column), type in a ticker, click Tab (as Codex suggested) or Enter and click "stock" in teh drop-down menu that pops up. There may be a few selections like listings in different countries. Also there may be "stock (smart)" (aggregates best bid and offer prices and sizes across exchanges and, unless configured ootherwise, trades are executed via IB's proprietary "smart routing") and "stock (direct)" (you can select exchanges from which you want to see quotes).
If you don't see what you want, under "Search by company name" you can type in a part of the company name and cliock "Find" button.
You can also right-click the desired row, in the menu click "Ticker..." type in ticker and further follow the same path as above.
I think most of TWS is intuitive enough without documentation. Just click or right-click everything and you fill fin 90% of the features (some may need to be anabled from "Configuration" before they are accessible). When you need documentation is when you want to understand how complex orders work and how to read reports (if you feeel you need to understand everything there).