First off, why are you using old fashioned VB junk from 2002 and earlier? That what the VisualBasic namespace is there for: to let VB6 programmers "move up gently" to .NET - it isn't meant to be used in "real applications".
You can use it, but adding a reference isn't enough - you also need to add the line:
using Microsoft.VisualBasic;
(As you are prompted at the line the error is on)
But stop using it, please. Throwing together a string input form takes what? 2 minutes, tops? And then you get the basis of a much more flexible validated input form that you can use in multiple applications...