DLookUp is a function which returns a value; you want to compare that
value with the value which is in your textbox. But there's actually a
simpler way: just see if the DLookUp returns nothing at all. Try:

Private Sub txtMyTitles_BeforeUpdate(Cancel as Integer)
Dim response As Integer
If Not IsNull(DLookUp("txtMyTitles","tblMyTitles","txtMyTitles = '" _
 & Me!txtMyTitles & "'") Then
 response = MsgBox("Title already exists", vbOKOnly)
 Cancel= True ' the BeforeUpdate event's Cancel property
              ' cancels the update if it's true
End If
' if Cancel is left False it goes ahead and does the update, no code
needed
End Sub

One tweak: if your title might contain an apostrophe ("Lady
Chatterly's Lover" for example) you can't use ' to delimit it. Safer
is to use ", ASCII value 34, as a delimiter:

If Not IsNull(DLookUp("txtMyTitles", "tblMyTitles",
"txtMyTitles = " & Chr(34) & Me!txtMyTitles & Chr(34)) Then

This all assumes that you have a Field named txtMyTitles in your table
(the first argument to dlookup) and that the table is named
tblMyTitles. If it were me, I'd use a form control named txtMyTitle
bound to a table field named MyTitle (singular not plural and without
a prefix), but whatever naming conventions work for you are fine.

                 John W. Vinson[MVP]