OK, let's say I have a PDF document called "Form.pdf" with 4 form fields. The form fileds are "name", "address", "postal_code" and "email".

Here's the code to create a new PDF file using "Form.pdf" as a template:

private void fillForm()
{
    string formFile = @"N:\.NET\Form.pdf";
    string newFile = @"N:\.NET\Filled-out Form.pdf";
    PdfReader reader = new PdfReader(formFile);
    PdfStamper stamper = new PdfStamper(reader, new FileStream(
                newFile, FileMode.Create));
    AcroFields fields = stamper.AcroFields;

    // set form fields
    fields.SetField("name", "John Doe");
    fields.SetField("address", "2 Milky Way, London");
    fields.SetField("postal_code", "XX1 4YY");
    fields.SetField("email", "johndoe@hotmail.com");

    // flatten form fields and close document
    stamper.FormFlattening = true;
    stamper.Close();
}

Acrobat 5 should be enough to create a PDF form. Let me know if you're having any troubles.