The following instructions are for OS X Leopard (10.5). For Snow Leopard (10.6), in both steps 2 below, change "Preferences" to "Preferences -> Accounts".
To have Mail search WatIAM as you type their name into the To field:
UW
uwldap.uwaterloo.ca
dc=uwaterloo,dc=ca
Now when you type a name in the To field of a new message, Mail searches the directories you have enabled and suggests a list of names that match what you typed.
To make your Address Book use WatIAM (UWDIR) Directory:
(You can skip steps 1-4 if you've done the steps above.)
UW
uwldap.uwaterloo.ca
dc=uwaterloo,dc=ca
To search WatIAM in Address Book, select Directories in the Group column and UW in the Directory column, and then type text in the search field.
To add someone from the network directory to your personal address book, drag the card from the directory to All in the Group column.
-- DanielAllen - 03 Mar 2011