Validating Item Numbers

Hello again...

I've been trying to use a vlookup function to see if I could validate the item number on the form but I'm getting stuck... the number is not being validated at all. I'm attaching a sample of the file I'm working on to see if anyone of you can tell me what I did wrong... or what I'm missing that is causing the problem. Please let me know if anyone has any suggestions or comments that would help find a solution.

And thanks again for any help anyone can provide.

Julian