SLAC supports eduroam site-wide. If you have that set up, connect to the wireless network called “eduroam” and you will be asked to authenticate in accordance with your institution's eduroam policy. Technical details can be found here: Eduroam Service at SLAC.
Alternatively, there is a “SLAC-VISITOR” wireless network that is open to all visitors to the lab. Select “SLAC-VISITOR” from the list of available networks, connect and follow the instructions to register as a visitor. More information can be found on the "SLAC-VISITOR" Wireless Network page.