In a Database model diagram you can set the cardinality by selecting the
relation (the acctual line between the tables), and then go to the Database
properties dialog box. In the Miscellaneous section you will find the
cardinality. To have a one to many, you need to set it to either Zero or more
or One or more.
Tip: Visio, by default, displays the relation as an arrowed line. If you are
used to the crows foot notation, you can change this, to better visualize the
cardinality of the relationship. On the Database menu, select Options and
then Document. Go to the Relationship tab of the Database Document Option
dialog, and select crow's feet.
Hope this was of help.
