Some routers require passwords in a particular format. Users may find that after entering a password for a WEP Internet connection, their AirPort menu bar icon shows an exclamation mark. You may also see an alert that reads, "Alert: No Internet connection..."
These routers actually have open networks that your Mac will associate with, but they still require a correctly formatted password to connect to the Internet. If the password is not interpreted correctly, the exclamation mark will appear and you will be unable to access content online.
To remedy the situation, you need to adjust the format of your Wi-Fi password. A Wired Equivalent Privacy (WEP) password is an older, less-secure password setup and should only be used if you have a device that only supports WEP encryption.
To join a WEP-protected network from your Mac you must enter one of two formats for your password. The first, ASCII, requires using a plain double-quotation mark (") before and after the password you were given to join the network. For example, if you are at a coffee shop that uses WEP encoding and they say their password is espresso, you would enter "espresso" to join the network.
The other format, hexadecimal, requires users to add a dollar sign ($) before the password. In the example above, you would enter: $espresso.
Most likely, your barista will not know which format the network is using, so just try both.