When someone is typing in a letter into a number field, how can I get an error message to pop up?

You would have to create your own custom Keystroke and Format JavaScripts. It would be easiest to copy most of the built-in one that Acrobat/Reader (and some other non-Adobe PDF viewers use), which you can find out more about by entering the following in the interactive JavaScript console, placing the cursor on the line of code, and pressing Ctrl+Enter:

AFNumber_Keystroke

It will spit out something looking like this:

function AFNumber_Keystroke(nDec, sepStyle, negStyle, currStyle, strCurrency, bCurrencyPrepend) {

    var value = AFMergeChange(event);

    var commit, noCommit;

    if (!value) {

        return;

    }

    if (sepStyle > 1) {

        commit = AFNumberCommaSepCommitRegExp;

        noCommit = AFNumberCommaSepEntryRegExp;

    } else {

        commit = AFNumberDotSepCommitRegExp;

        noCommit = AFNumberDotSepEntryRegExp;

    }

    if (!AFExactMatch(event.willCommit ? commit : noCommit, value)) {

        if (event.willCommit && !event.silenceErrors) {

            var cAlert = EScriptString.IDS_INVALID_VALUE;

            if (event.target != null) {

                cAlert += " [ " + event.target.name + " ]";

            }

            app.alert(cAlert);

        } else {

            app.beep(0);

        }

        event.rc = false;

    }

    if (event.willCommit && sepStyle > 1) {

        strval = event.value;

        commas = new RegExp;

        commas.compile(",");

        strval = strval.replace(commas, ".");

        event.value = strval * 1;

    }

}

You can either modify this function to deal with keystrokes that it disallows and place it in a document-level JavaScript, or leave it as-is and add code that's called before you call this function to deal with the non-numeric keystrokes.

You will also need to call the same Format script that Acrobat/Reader uses. I realize all of this may be a bit confusing, so if you need more help or sample PDF that demonstrates this, post again.