In Mac Mail, select the Mail -> Preferences...
Click on the Composing button at the top then select Configure LDAP...
Next, fill in the boxes correctly:
Name: CSULB LDAP
Server: ldap.csulb.edu
Search Base: o=csulb
Port: Place a check mark on the box labeled Use SSL then change the port to 389
Username: uid=username,o=csulb
Password: password
Click Save when you are done.
Then, go to File -> New Message.
Type the name of the individual in the To: field and the email address of the matching people will come up.