From your auto attendant, direct callers to dial "*2" (that's "star-two"

to access the dial by name directory. You determine whether it's first name or last name when you put the names in, so you might say "To use the Dial-by-FIRST-Name Directory, press Star Two" (or Last Name).
To put the names in, log into the system administrator's mailbox, press 9 for programming and then 2 for extensions. From there, enter the extension number and #, 1 for directory listing or recorded name, 2 for directory listing, and 1 to enter a new directory listing.
Now, using the keypad, enter up to 10 letters. There is a chart in the manual, but here's how it works. Figure it out, and you'll remember it for a life time.
Let's say I want to put in "Matt" as the name. For M, press 61. 6 because the M is on the 6 button, and 1 because it's the 1st letter on that button. For A, press 21, (it's on the 2 button, as the 1st letter), for T press 81, and for the last T press 81 again. See the pattern?
Press # at the end, the letters will be read back, and press *# if it is correct.