This main window button adds a new attribute to the currently selected object. If you use it, the Create New Attribute dialog is shown. You can choose here the name of the attribute, the data type (also called attribute syntax) and of course the value:
If you use the drop down menu you can choose one of the attributes which are known from the directory schema. LEX shows you automatically the correct attribute syntax in this case. Please note that this functions are only available if the LDAP schema is evaluated correctly from the server:
By the way: If you see the Add new attribute button disabled and gray, then your LEX is probably in Read Only mode.
To create a new attribute, you can also press the INSERT key or use the menu option Edit - Add New Attribute when the focus is set to the attribute list.
Another way to create attributes is to deactivate the Show only attributes that have values button . Then all the attributes are shown that your currently selected object can have. To create an attribute which was not set for the object before, just select the regarding attribute and double-click it, or press F2 or ENTER to edit its value: