The mysql.sock file contains information on how to communicate with mysql. It is created whenever the mysql daemon is started.
You say the file /tmp/mysql.sock exists. Is it a link? If so, where does it
go? Since you say you didn't install the MacPorts version of mysql, the
shouldn't exist. Did these commands produce an error when you ran the
script? If myql.sock exists and isn't a link to the MacPorts location, then it was probably put there when mysql started up.
The most important question is whether or not you can communicate with mysql. I suggest trying to connect on the command line. If you can connect to mysql, then you don't need to worry about this and can proceed as detailed in the WeBWorK installation manual.
If you can't communicate with mysql, then you need to figure out why you can't. Did you install the mysql Startup Item? How about the MySQL preference
pane? Either of these should help you start mysql up.