You could do something like this:

if (event.value > 3 && event.value < 10) {
    event.rc = false;
    app.alert({
        cMsg: "Please contact Larry at xxx to process your form.",
        cTitle: "My Window Title",
        nIcon: 0,
        nType: 1
    });
}

You can enter this validation script by editing the properties of a field. Go to "Validate Tab", click "Run custom validation script", then "Edit...". Type the code into the JavaScript Editor window, and then click "Ok" and "Close".

(source: skitch.com)

I would recommend the JavaScript guide published by Adobe. It contains examples and detailed instructions on what you can and can't include in a pdf (with regards to JavaScript).

http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/acro7jsguide.pdf

If you are going to do this for a lot of documents, please consider using iText. I can certainly imagine a corporate workflow where you want to generate these kinds of forms automatically and provide all of them with the required validation. In which case doing it by hand is going to be a bottleneck.

For more information about how to do this with iText, have a look at

http://developers.itextpdf.com/examples/itext-action-second-edition/chapter-7