Kristine, a multi-select list box is not the choice if you want to store
values in the table.

Instead, you need to create a related table (one-to-many relation) so you
can store multiple records matching the one in your main table. You can then
use a subform, and add as many rows as apply to the record in the main form.
You can do this without writing any code.

Even if you were quite at home writing code, I would still suggest that this
normalized approach is the way to go.