The dialog itself functions properly, however there are a few scripting things you could change. /loadbuf -ro eliminates your /did -r; take a look at the helpfile and the /loadbuf switches a bit more. Your lack of error checking is not a good thing. You need to check things like:

If the user has selected an item before deleting it.
If the user has entered text into the textbox before adding it.
If the dialog is in use.

Not to mention, the concept and script is rather simple... I think you should have worked this out a little more before release.