Just ask for their network/username, and password. Then it's usually as simple as clicking on the wireless strength symbol, on your toolbar. That will display the networks available. Click on the one for their wi-fi, click connect. If one is set up, you will be asked for their password. Then you're "in". Some places have a more complex "set-up", but they are usually happy to show you how, or do it for you (assuming they're not really busy.) Most are very helpful.
Do bear in mind, they are public networks. Avoid doing sensitive transactions, especially "financials".