Hope this helps.
To select an item for a listbox do the following:
Populate the listbox as you normally would.
After populating try the following, to which you'll need to supply the listbox name as indicated by yourListBoxIDhere
yourListBoxIdHere.SelectedItem = yourListBoxIdHere.Items.IndexOf(yourListBoxIdHere.Items.FindByValue("WhatYouAreLookingFor"))
The only thing I don't know is how to get it to select multiple values, should you allow that. Also, most likely WhatYouAreLookingFor is probably going to be an int or Integer since you might have it as an index value of some sort from your database.
Hope that helps.