I see that you mentioned you're using the Plugins Generator for HTML-Kit 292. Plugins Generator doesn't provide a quick way to create dialogs where you could select a snippet to insert. If you're familiar with how to do that in another programming language, HTML-Kit 292 provides an interface to call such programs.
If what you're wanting to do is bring up a dialog of snippets, select a snippet and insert it in the editor, then that functionality already exists in Ctrl+Shift+I shortcut. "File > Save Selection > As Snippet" menu option can be used to add items to that dialog.