Working with Listboxes

This is probably fairly simple but for some reason it's taking me awhile to figure it out so I'm going to just ask the pros...

I'm looking to fill a listbox after the user selects from another listbox in the PMP (Property Manager Page) for an Addin. 

From what I can tell the 'UserPMPage.vb' handles setting up the PMP and the 'PMPHandler.vb' is where code would go to react to making a selection of the first listbox.

I don't know how to go about refreshing the PMP page after a selection from the listbox is highlighted. 

SolidworksApi macros